Prepare the Oven and Pans: Preheat the oven to 350°F. Line 24 regular cupcake cups with paper liners, 48 mini cupcake cups, or use a combination of both sizes.
Cream the Butter and Sugar: Place the softened butter and granulated sugar in a large mixing bowl. Beat on high speed for 3–4 minutes, until the butter becomes light and fluffy.
Add the Melted Chocolate: Pour in the melted semi-sweet chocolate and beat for about 1 minute, until it is thoroughly incorporated into the butter mixture.
Blend in the Eggs and Vanilla: Add the eggs and vanilla extract. Beat for another 3–4 minutes, scraping down the sides of the bowl as needed, until the mixture is very smooth and creamy.
Prepare the Coffee Milk: Warm the whole milk in the microwave for about 1 minute, just until lukewarm. Stir in the instant coffee until completely dissolved.
Add the Coffee Milk: Pour the coffee-infused milk into the batter, but do not stir immediately. Let it stand while you prepare the dry ingredients.
Combine the Dry Ingredients: In a small bowl, whisk together the all-purpose flour, cocoa powder, baking powder, and salt.
Sift the Dry Ingredients: Sift the dry mixture directly into the bowl containing the wet batter.
Finish the Cake Batter: Mix on medium speed for 1 minute, just until a smooth batter forms.
Fold in the Chocolate and Toffee: Add the toffee chips and semi-sweet chocolate chips. Fold them into the batter by hand until evenly distributed.
Fill the Cupcake Liners: Use a measuring scoop or spoon to portion the batter into the prepared liners, filling each one about ¾ full.
Bake the Cupcakes: Bake regular cupcakes for 24–25 minutes or mini cupcakes for about 20 minutes. The cupcakes are ready when a toothpick inserted into the center comes out clean.
Cool the Cupcakes: Remove the cupcakes from the oven and allow them to cool completely before adding the frosting.
Whip the Butter: For the caramel buttercream, place the softened unsalted butter in a mixing bowl. Beat on high speed for about 5 minutes, scraping down the sides frequently, until the butter is light and fluffy.
Add the Dulce de Leche: Add the dulce de leche to the whipped butter and mix until incorporated. If dulce de leche is unavailable, a similar caramel sauce can be used. If the frosting needs additional thickness, increase the amount of confectioner’s sugar.
Season the Frosting: Add the vanilla and begin with ¼ teaspoon of fine salt. Add more salt as needed to adjust the flavor.
Add the Confectioner’s Sugar: Add the sifted confectioner’s sugar 1 cup at a time, scraping the sides of the bowl frequently so everything blends evenly.
Whip the Buttercream: Once all the confectioner’s sugar has been incorporated, beat the frosting on high speed for 2–3 minutes, until it becomes very fluffy.
Prepare the Piping Bag: Transfer the finished buttercream to a pastry bag fitted with a star tip. The original recipe uses an Ateco #829 tip.
Frost the Cupcakes: Pipe generous swirls or dollops of caramel buttercream over each completely cooled cupcake.
Add the Garnishes: Sprinkle the frosted cupcakes with additional toffee chips and drizzle with caramel sauce, if desired.
Store the Cupcakes: The cupcakes can remain at room temperature for up to 24 hours. For overnight storage, refrigerate them and then allow them to sit at room temperature for 1 hour before serving.
