Place the butter in a mixing bowl. Zest half of the orange directly into the bowl (about 1 tablespoon). Use a mixer on high speed to beat until light and fluffy, about 2 minutes.
1 cup salted butter, 1 medium Navel orange
Sift the powdered sugar and cocoa into the bowl. Mix on low speed to combine. Add the milk, 1 tablespoon at a time, until you reach your desired consistency. You may not need all of the milk. Add the vanilla and give the frosting a final mix.
If you don't have an orange, you can use orange extract in a pinch.This recipe makes enough to frost one 8-inch double layer cake.You can make this recipe ahead and freeze it for later. When ready to use, thaw at room temperature.