8 Tbsp. unsalted butter
1 1/2 c. superfine sugar
1 1/2 c. cocoa powder, preferably Dutch-processed
Pinch of salt
1 c. heavy cream
1 tsp. instant coffee crystals (optional)
1/2 c. sour cream
1 1/2 tsp vanilla extract
To make the frosting, melt the butter in a large saucepan.
Stir in the sugar, cocoa, and salt.
The mixture might look grainy or sandy and this is fine.
Whisk the cream and instant coffee into the cocoa mixture and cook over low heat, while stirring to dissolve the sugar grains.
The mixture should get hot to the touch but never simmer or boil.
Remove from the heat and stir in the sour cream and vanilla.
Let cool until the frosting thickens an becomes spreadable, about 1 hour.
If it seems too thin, stir in some powdered sugar.
Remove the cupcakes from the tins. Spread the frosting on the tops of the cupcakes.