Preheat oven to 350 degrees (F). Line a 12 cup muffin tin with paper liners and set aside.: The kitchen immediately warms with a subtle toasted scent that hints at the transformation to come, and the sound of the oven clicking on is oddly reassuring. Getting the oven to a full steady 350 degrees ensures even rise and consistent bake times, so always allow a few minutes for the temperature to stabilize before sliding the tin in. If the oven is too cool, cupcakes will bake longer and may be dense, while an oven that runs hot can brown the tops before the centers set. A common mistake is skipping liners or not arranging them before filling, which makes portioning messy and slows preparation. I like to line the tin first to visualize how many cupcakes I will fill and to keep the batter from sticking to the pan edges.
In a medium bowl combine 1 and 3/4 cups of the flour, baking powder, baking soda, and salt; set aside. In a separate bowl combine the sour cream, milk, and 2 tablespoons of the lemon juice; whisk well to combine and set aside. Place blueberries in a small bowl; add remaining lemon juice and toss to coat the berries; add remaining flour and toss to coat the berries in the flour; set aside.: The dry mixture smells faintly of grain and baking powder, a neutral base waiting for flavor. The lemon and dairy mixture will look silky, with tiny bubbles where the lemon reacts to the dairy, which slightly tenderizes the batter. Tossing blueberries in a little flour keeps them suspended in the batter rather than sinking to the bottom, which helps each cupcake get fruit in every bite. When you coat berries in flour, you may see a dusting that seems excessive at first but disappears once folded in. Avoid overmixing the sour cream into the dry ingredients later, because that can make the crumb tough; gentle folding preserves a tender texture.
In a large bowl using a handheld electric mixer, or in the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until smooth and creamy, about 30 seconds. Add in the granulated sugar and lemon extract (if using) and beat until well combined. Add in the egg and egg yolk and mix well. Reduce the mixer speed to low. Add in 1/2 of the flour mixture and mix until just combined (this should take about 5 seconds). Add 1/2 of the sour cream mixture and mix until just combined. Repeat with remaining flour and sour cream mix, being sure to mix on low until just combined. Using a rubber spatula, fold in the floured blueberries (and any excess flour in the bowl) and lemon zest.: As you cream butter and sugar , you will notice the mixture lighten and become ribbon like when the beaters are lifted, which traps air and helps the cupcakes rise. Adding eggs integrates moisture and protein that strengthen the structure. Alternating dry and wet ingredients prevents overworking gluten, so the crumb stays tender; you want to stop mixing as soon as you do not see streaks of flour. When folding in the floured blueberries be gentle and use a spatula to avoid bursting the berries, which would tint the batter blue and concentrate juices. A typical pitfall here is overmixing once the flour is added, which develops gluten and yields a chewy cupcake rather than a delicate one. Watch the batter texture; it should be thick but scoopable, dotted with berry and flecks of lemon zest.
Spoon the batter into the prepared muffin cups, filling them 3/4 of the way full. Bake for 16-20 minutes, or until a toothpick inserted in the center comes out clean. Allow cupcakes to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely. Once cupcakes are cool, frost and decorate. The cupcakes may also be frozen, unfrosted and stored in a freezer safe bag, for up to 2 months.: As you portion batter into liners, you will hear a soft steady plop and see domes forming as the oven heat does its work. The tops should turn a soft golden hue and spring back slightly when pressed. Insert a toothpick near the center within the recommended time frame to avoid overbaking, which dries the crumb. Letting cupcakes rest briefly in the pan makes them easier to remove without breaking, but transferring them to a wire rack cools them evenly so condensation does not collect under the liner. Avoid frosting warm cupcakes, or the frosting will melt into the crumb. If you plan to freeze, make sure they are completely cool and wrap tightly to prevent freezer burn. A common mistake is opening the oven too often during the bake, which can cause uneven rise.
In a medium bowl using a handheld electric mixer, or in the bowl of a stand mixer fitted with the paddle attachment, beat the butter and cream cheese on medium speed until smooth and creamy. Reduce the speed to low and gradually add in the powdered sugar and lemon juice, beating until light and fluffy, about 2 minutes. Add in the vanilla extract, lemon extract (if using) and lemon zest; beat smooth. If the consistency appears too thick, add a teaspoon of milk or heavy cream, but only if necessary. The frosting should be creamy and spreadable, but also thick enough to hold its shape. Pipe or spread the frosting on top of each cooled cupcake.: The cream cheese and butter should blend into a silky paste before the sugar goes in, which prevents lumps. Adding confectioners' sugar gradually helps it incorporate smoothly without a cloud of sugar in your kitchen. The frosting should taste bright from the lemon juice and zest, with a balanced sweetness. If it becomes too loose, chill it briefly until it firms up; if it is too stiff, a teaspoon of milk will loosen it without making it runny. Avoid overbeating after the sugar is added, as this can aerate the frosting too much and cause it to break or weep. A handy test is to lift a spoonful and see if it holds a soft peak.
Top each frosted cupcake with a pinch of lemon zest and extra blueberries, optional but pretty!: The final garnish adds texture and an aromatic lift that signals the flavors inside. Fresh blueberries on top provide a cold burst against warm cake and creamy frosting, while a little lemon zest brightens the aroma when you lean in. Keep the decoration simple to avoid overloading the cupcake, and place blueberries gently so they do not sink into the frosting. One mistake I see is adding too many berries, which can make serving messy and the presentation heavy. A light sprinkle keeps the look elegant and the bite balanced.
