While the cupcakes are in the oven, make the frosting. (There is cooling time involved.)
Melt the butter in a large saucepan over medium heat. Stir in sugar, cocoa, and salt. The mixture will be thick and grainy.
Whisk heavy cream, sour cream, and instant coffee in a large measuring cup. Slowly add the cream mixture into your saucepan and whisk until smooth. Cook over medium-low heat until the sugar has dissolved and is hot to the touch, between 4-8 minutes. (Mine cooked in about 4 minutes, but it could take longer.) Do not boil!
Transfer frosting to a bowl and let cool 2-3 hours until it’s spreadable. You can also chill it so it will set up faster. If it gets to hard to spread, microwave it on 20 second intervals, stirring in between, until it’s the desired consistency.
Frost top of the cupcake with chocolate frosting. Store in refrigerator, let come to room temperature before serving.