Preheat the oven to 350F. Line 24 regular cupcake tins with paper liners, or 48 mini cupcakes tins; or do a combination of both. For the cake batter: In a large mixing bowl, combine the softened butter and sugar. Whisk on high speed for 3 to 4 minutes, until the butter is light and fluffy. Add in the melted chocolate and whisk again for about a minute until it's well combined. Add in the eggs and vanilla and whisk again for another 3 to 4 minutes, until the batter is very smooth and creamy. Scrape down the sides of the mixing bowl often to ensure everything gets well combined.

Warm the milk in the microwave for about a minute, just until it's lukewarm, then add in the instant coffee and stir until the coffee is dissolved. Add the coffee milk into the batter but do not stir just yet; let it stand. Meanwhile, prepare the dry ingredients: combine the flour, cocoa powder, baking powder and salt in small bowl and mix together. Sift the dry ingredients into the cake batter, then mix on medium speed for 1 minute, until a smooth cake batter forms.

Fold in the toffee chips and chocolate chips by hand last. Using a measuring scoop or spoon, fill the cupcake liners almost to the top, about 3/4 full. Bake the cupcakes in the preheated oven for 24 to 25 minutes for regular size; about 20 minutes for the mini cupcakes. Bake until a toothpick inserted into the center comes out clean. Allow the cupcakes to cool completely before frosting them!

For the Caramel Buttercream: Place softened, unsalted butter into mixing bowl. Whisk on high speed for about 5 minutes, until butter is light and fluffy. Scrape down sides of mixing bowl often to get all the butter mixed well. Add the dulce de leche (cooked sweetened condensed milk). If you cannot find dulce de leche, use a similar caramel sauce. If necessary, increase the amount of confectioner's sugar to thicken the frosting.

Add the vanilla and salt. Start by adding 1/4 teaspoon salt, then add more if needed. Begin adding the confectioner's sugar 1 cup at a time, scraping down sides of mixing bowl often to get everything incorporated. Make sure to sift confectioner's sugar before adding to avoid any clumps. After adding all sugar, whisk on high speed for 2 to 3 minutes to get frosting very fluffy.

Transfer the frosting into a pastry bag tipped with a star tip; I used Ateco tip #829. Pipe generous dollops of cream on top of each cupcake and garnish with more toffee chips and a drizzle of caramel sauce, if desired. Cupcakes can stand at room temperature for 24 hours; I recommend refrigerating them overnight. If refrigerating, allow the cupcakes to warm at room temperature for 1 hour before enjoying again.