Preheat oven to 350°F.: The kitchen fills with a warm dry scent that signals browning to come, and starting with a fully heated oven ensures the crust begins to set immediately for a firm base. Preheating is critical because if the oven is too cool the crust can bake unevenly and become greasy instead of crisp. A common mistake is skipping preheat, which leads to underbaked centers, so always wait until the oven thermometer reads steady temperature.
Lightly grease a 9 x 13 baking dish or line with parchment paper.: Oiling the pan or lining it with parchment makes removing the bars effortless, and the parchment also protects the edges from overbrowning. The shallow, nutty smell of warm pan rubbed with butter is comforting, and it sets the stage for even heat contact across the crust. Avoid using only cooking spray on parchment, as it can cause sliding when you try to transfer, which is a frequent slip up.
Combine flour, salt, oats, coconut, and brown sugar in a large bowl of electric mixer and mix at low speed.: As the dry mix blends you will notice a toasty coconut aroma and the brown sugar beginning to coat the oats, creating a cohesive base. Mixing at low speed prevents flour clouds and helps distribute the salt evenly for balanced flavor. Over mixing can break down the oats and make the texture too fine, so keep the speed gentle for a crumbly, rustic blend.
Add butter and continue mixing until a crumbly dough forms. Add egg and vanilla extract and continue mixing until combined. Stir in chocolate and butterscotch chips.: When you add the butter , the mixture will clump into coarse crumbs and give off a rich buttery scent. Incorporating the egg and vanilla extract brings shine and elasticity to the dough without making it runny. Folding in the chocolate chips and butterscotch chips distributes flavor pockets across the dough. A common error is overworking here, which can warm the dough and cause chips to smear, so mix just until they are evenly scattered.
Reserve 1-1/2 cups of dough for topping. Press remaining dough into bottom prepared baking dish. (Sprinkle on some more chocolate chips if desired.): Pressing the base creates a compact crust that browns pleasantly, and you will feel a slight resistance as the crumbs come together under your fingers. The surface should be even and moderately firm to hold the filling. Forgetting to reserve enough dough for the topping is a typical oversight, so measure out the reserved portion before pressing to ensure the top is substantial.
Bake for 10 minutes.: During this short bake the edges will set and the crust will start to take on a light golden tone, releasing a toasty scent. This brief initial bake prevents the crust from becoming soggy when you pour on the filling, and it creates a slight barrier that helps the bars hold shape. Leaving it in too long can dry out the base, so stick closely to the time and peek for a gentle golden hue.
Pour sweetened condensed milk evenly over the hot crust.: The glossy stream of sweetened condensed milk smells rich and slightly caramel like as it spreads, and pouring while the crust is hot helps the milk settle into crevices for an even gooey layer. Distributing it carefully prevents pooling in one area which can cause uneven texture after baking. A common mistake is pouring too quickly, which can overflow the pan, so pour slowly and move across the surface.
Top evenly with the reserved cookie dough.: As you scatter the reserved dough, you will see soft islands of crumb that will brown into a textured top. Press lightly only in spots where pieces are loose, leaving some crumbs to remain airy and golden. This balance between compact and loose topping creates the signature chew and crisp contrast. Try not to press everything flat, because that flattens the visual appeal and reduces textural variety.
Bake 25 to 30 minutes until golden brown. Cool and cut into bars.: The oven will perfume the kitchen with toasted coconut and caramelized sugar scents, and the top should be a warm golden brown without dark spots. Cooling is essential so the filling firms slightly, giving clean slices and a stable chew. A classic pitfall is cutting while too hot which results in messy, sinking squares, so allow adequate cooling time for the best presentation and texture.
Combine chocolate chips and oil in a small bowl. Microwave in 30 second increments on high heat, stirring in between, until fully melted. Drizzle over top of cooled bars. Let set up before serving.: Melted semi sweet chocolate chips with a touch of vegetable oil or coconut oil becomes glossy and pourable, adding a luxurious finish and a slight snap once set. Drizzling creates elegant lines and a concentrated chocolate hit that cuts through the sweetness. Be cautious not to overheat the chocolate, as it will seize and become grainy, which is the most common mistake at this stage. Stir gently until smooth, then decorate and allow it to firm before slicing.
