4 tablespoons unsalted butter
3 tablespoons unsweetened cocoa powder
1/4 cup whole milk
1 teaspoon vanilla extract
2 cups powdered sugar
Melt the butter in a large microwave-safe bowl. Add the cocoa powder and whisk to combine. Add the milk and vanilla, and whisk until frothy. Make sure the cocoa powder has completely dissolved. Beat the powdered sugar into the cocoa mixture a little at a time until fully incorporated. The frosting should be thick but spreadable. If runny, add additional powdered sugar; if too stiff, work in a little more milk.