½ cup / 120 ml heavy whipping cream plus more, as needed
1 cup / 220g unsalted butter
1 tsp espresso powder
1 pinch kosher salt
16 oz / 450g bittersweet chocolate coarsely chopped
2½ cups / 300g confectioner's sugar sifted if lumpy
Instructions
In a medium saucepan over low heat, combine the cream, butter, espresso powder, and salt and bring to a gentle simmer, stirring often until the butter is melted. Add the chocolate and swirl the pan until the chocolate is submerged in the hot cream. Let the mixture sit for about 3 minutes, then whisk until smooth.
Pour the mixture into the bowl of a stand mixer fitted with the paddle attachment. Let the mixture cool to room temperature; it will become quite thick as the butter and chocolate set.
Turn the mixer speed to low, add the confectioners' sugar, a little at a time, and beat until smooth and spreadable. If the frosting is at all grainy, add additional cream, 1 Tbsp at a time, until smooth. The frosting will be quite soft but firm enough to cling to the cake and spread evenly.
Use immediately.
Notes
On a hot day, the mixture may not thicken adequately. You can pop the bowl in the fridge for several minutes before adding the confectioners' sugar.