If you are using instant coffee, combine 2 tsp with 2 1/2 tsp of really hot water and whisk to combine. Allow to cool a bit before starting your frosting.
In the bowl of a stand mixer, beat the butter on medium-high until it is soft and smooth.
1/2 cup butter
Reduce the speed to low and slowly add the powdered sugar 1/3 cup at a time. Allow the powdered sugar to incorporate before adding the next 1/3 cup. When you've added all the powdered sugar, beat on medium for a minute.
2 1/4 cup powdered sugar
Reduce the speed to low and repeat with the cocoa powder 1/4 cup at a time using the same method you used with the powdered sugar.
3/4 cup unsweetened cocoa powder
If you're using espresso powder, you can mix it with the vanilla extract and add it now. Or add the dissolved and cooled instant coffee.
2 tsp espresso powder, 1 tsp vanilla
With the mixer on low, slowly add the milk until the frosting starts to smooth out. Increase the speed to medium-high and whip the mixture until it is buttery smooth.
1/4 cup milk
Notes
Recipe will make enough buttercream for 12 cupcakes. You can adjust the number of servings in the interactive recipe card above.In step 3, the mixture is going to be really crumbly. Don't worry, stay with me. It's going to be amazing really soon.Store in an air-tight container in the fridge for up to a week.