In a large mixing bowl, beat the softened butter with a hand mixer or stand mixer until smooth and creamy (about 2 minutes).
Add the powdered sugar and cocoa powder, one cup at a time, beating on low speed until mostly combined (to avoid a cocoa cloud!).
Slowly drizzle in the milk and vanilla. Beat on medium-high speed until light, fluffy, and spreadable—about 2–3 minutes.
Check the consistency:
If too thick → add a splash of milk.
If too thin → add 2–3 tablespoons more powdered sugar.
Spread generously over a cooled cake or pipe onto cupcakes.
Enough to frost a 9×13 cake or about 18–24 cupcakes.
