Make the Frosting
- Cook the Cocoa Frosting:
In a medium saucepan, combine milk, cocoa powder, and butter. Bring to a gentle boil, stirring constantly. Once it reaches a boil, remove from heat. - Finish and Spread the Frosting:
Stir in the confectioners’ sugar and vanilla extract. Beat the mixture until smooth, then fold in the chopped walnuts, if using. Immediately pour the warm frosting over the warm cake and spread quickly to cover evenly.
Serving and Storage
Let the cake cool at room temperature to set the frosting before slicing into squares. This cake is best served at room temperature and keeps well covered on the counter for 2–3 days. For longer storage, refrigerate and bring to room temp before serving.
