Preheat oven to 350F. Line a 12-cupcake pan with paper liners.
In a large mixing bowl, mix melted butter with sugars and cocoa powder.
Add vanilla, and eggs to your mixture. Whisk, whisk, and whisk some more! (note: the more you whisk the sugars with the egg - the more crackly your brownie surface will be after baking!)
Add flour, espresso powder, and salt. Mix until all flour has been incorporated. Mixture will resemble a thick, glossy, paste. Mix in your semi-sweet chocolate (as desired).
Equally distribute brownie mixture into 12 lined cupcake moulds.
Prepare 12 x 1/2" tin foil spheres (note: I do this by cutting an approximately 3"x6" rectangle out of tinfoil and crumpling it into a ball. This is a rough measure! No need to be precise here :).
Place one tinfoil ball between the cupcake mould and paper liner of each of the 12 cupcake moulds. The placement of each ball should create a heart appearance.
Bake brownie cakes into a preheated oven for 18-24 minutes or until a toothpick inserted into the centre shows moist crumbs attached (note: the brownies will continue to bake in the cupcake pan after removing from the oven so having a bit of batter on the toothpick is fine! Just make sure the edges are set). Cool in a cupcake tin until firm enough to be held with one hand and then cool completely on wire rack.
Clean/dry your large bowl and you are ready to prepare your frosting while the cupcakes are cooling!