Preheat the oven to 350 degrees (F). Line a 12-cup cupcake/muffin tin with cupcake liners and lightly spray the liners with non-stick spray (optional but it does help them peel right off).: The kitchen will warm and you will notice the air slightly fragrant with anticipation as the oven climbs to temperature, which ensures a steady environment for an even rise. Proper preheating produces consistent domes and predictable bake times, so resist putting the cupcakes in before the oven reaches 350 degrees F . A common mistake is relying on the oven light instead of a thermometer, which can cause underbaked centers; if in doubt use an oven thermometer.
Melt the oil, butter, and chocolate together in the microwave, heating in 30 second increments, and stirring between increments each time. You can also melt the oil, butter, and chocolate over very low heat on the stovetop, but I find the microwave to be much easier. Whisk mixture until completely smooth and set aside to cool.: A gentle, rhythmic stirring while melting the coconut oil , butter , and semi sweet chocolate chips makes the mixture glossy and silky. You will notice the aroma shift from fatty to deeply chocolatey as the chips dissolve, and the mixture should appear uniform with no streaks. This melt forms the backbone of your batter, delivering moistness and shine. Overheating will burn the chocolate and introduce a bitter note, so heat in short bursts in the microwave or use a very low simmer if using the stovetop.
In a medium sized bowl combine the flour, baking soda, baking powder, cocoa powder, and salt; stir together until thoroughly combined; set aside.: When these dry ingredients are mixed, you should be able to see an even, deep brown blend with no pale pockets, which prevents uneven leavening and cocoa clumps. Sifting or whisking at this stage helps aerate the flour and evenly distribute the chemical leaveners. A frequent error is adding wet ingredients directly to a poorly mixed dry base, which can result in uneven texture and pockets of unreacted leavening causing odd domes.
In a large bowl, whisk together the eggs, yolk, sugar and vanilla together; beat until smooth. Add the cooled oil/butter/chocolate mixture and whisk until smooth. Add half of the flour mixture, then half of the sour cream. Repeat the process until everything is added, and be sure to mix until JUST combined - then quickly stir in the hot coffee/water. It's important not to over mix, here! Just stir until evenly combined.: Whisking the eggs , extra egg yolk , granulated sugar , and vanilla until smooth creates a cohesive emulsion that traps a little air for lift, producing a lighter crumb. You'll see the mixture lighten slightly and become glossy, which indicates proper incorporation. This step helps the batter hold structure without overdeveloping gluten. If you whisk too aggressively you can overwork the proteins, so stop when smooth and slightly glossy.
Divide the batter among the 12 liners in your prepared pan, filling each cup 3/4 of the way full (you will have enough batter leftover to bake 4 more cupcakes). Bake for 16-18 minutes, or until a toothpick inserted in the center comes out clean. Cool completely before stuffing with Nutella and frosting. Once cooled, use a small sharp knife to carve out a small hole in the center of each cupcake; do not discard the carved out cake piece. Fill each hole with two teaspoons of Nutella, then cover again with the carved out cupcake piece. Set aside and make the raspberry frosting.: Folding in the cooled chocolate mixture followed by alternating additions of the dry mix and full fat sour cream results in a tender, well hydrated batter. After adding each component, stir until it is just evenly incorporated; the surface should look smooth and slightly glossy, not flat or gummy. The hot coffee or hot water at the end blooms the cocoa and thins the batter to a pourable consistency. Overmixing in this phase will activate gluten and create dense cupcakes, so mix gently and stop when evenly combined.
Combine the raspberries and lemon juice in a small saucepan; bring to a simmer over medium heat. Once simmering, stir the mixture occasionally, breaking up the berries with a wooden spoon, until the raspberries have become soft enough to mash effortlessly. It will look like raspberry jam. Once the mixture can coat a spoon (you should have about 1/4 cup of raspberry puree) remove from heat. Allow mixture to cool completely before using. <--- That part is SUPER important!: As you fill each liner, you will notice a batter that slowly settles but keeps a domed top when baked. Filling to three quarters allows room for rise and prevents overflow. If you underfill, the cupcakes will be small and flat; if you overfill, they will spill and bake unevenly, so aim for consistent portions, using a scoop if helpful.
Place the softened butter in a large bowl or in the body of a stand mixer. With a handheld mixer, or with the paddle attachment on your stand mixer, beat butter on medium-high (scraping down the sides as needed) until completely smooth; about 2 minutes. Add in the raspberry puree and beat until completely combined; about 1 minute (don't worry if the mixture looks weird at this point). Add in the confectioners sugar, one cup at a time, on low speed. Once all of the sugar has been added, add in the cream and salt, then beat on high-speed for a full 2 minutes - this thickens up the frosting and makes it nice and fluffy. Frost cooled cupcakes as desired.: During baking the cupcakes will rise and their top will spring back slightly when touched, and the aroma of baked chocolate will deepen. The toothpick test ensures the crumb is set, avoiding underbaked centers. Pull them too soon and you risk a gummy interior; leave them too long and the cupcakes dry out, so watch near the minimum time and adjust for your oven.
Cool completely before stuffing with Nutella: Cooling brings the crumb to room temperature and firms the internal structure so you can carve without tearing. The cupcakes will stop steaming and the internal juices will redistribute, preventing melted fillings from sinking or bleeding. A common mistake is trying to fill while warm, which causes the Nutella to melt and leak, so be patient and cool fully.
Carve a small hole and fill with two teaspoons of Nutella: The act of carving should reveal a soft, tender crumb that yields easily to a small knife; the reserved cake piece will fit back as a lid for a neat finish. Filling with two teaspoons of Nutella gives a wonderful surprise without destabilizing the cupcake. Overfilling will make them messy and difficult to frost, so measure the filling carefully and tamp the lid back in place gently.
Simmer raspberries with lemon juice to make a puree: As the raspberries soften in the saucepan and break down, they will release a vivid ruby syrup that smells bright and fruity, resembling a jam consistency. Reducing until the mixture can coat a spoon concentrates the flavor and removes excess water that would thin your frosting. Cooling is essential; adding hot puree to butter can melt it, so wait until it is room temperature to avoid runny frosting.
Beat softened butter then add raspberry puree: Whipping the VERY soft butter yields a silky, billowy base that becomes light after adding the cooled puree. The bowl will sound smoother as air is incorporated, and the color will shift toward a pale pink depending on the fruit concentration. If the butter is not soft enough, you will see lumps; if it is too warm, the mixture will separate, so achieve the proper softness before starting.
Add confectioners sugar cream and salt then beat high: Adding the sifted confectioners sugar gradually on low speed prevents clouds of sugar and ensures a smooth integration, while raising the speed later makes the frosting fluffy and stable. The final high speed whip should yield a glossy, pipeable buttercream that holds peaks. If the frosting is gritty, you likely didn't sift the sugar or beat it long enough; if it is too thin, add more sifted sugar sparingly.
Frost cooled cupcakes as desired: As you pipe the buttercream the texture should be creamy and hold shape, and the scent of raspberries will be fresh and bright. Decorating is the final creative step where you can add a fresh raspberry or simple swirl, making each cupcake inviting. A common pitfall is frosting warm cupcakes, which will cause the buttercream to soften and slide off, so ensure everything is cooled.
