Add butter to bowl of stand mixer. Using paddle or whisk attachment, beat until light and fluffy.
Add the cocoa powder and powdered sugar. Mix on low speed to combine. With the mixer running, add the evaporated milk and the vanilla.
Once combined, increase speed to high and blend until fully mixed and smooth. Frosting will be thick. Add additional evaporated milk, 1 tablespoon at a time, to thin if needed.
Notes
Recipe makes enough to frost one double layer 9-inch round cake or 24 cupcakes
Although I use unsalted butter for this recipe, you can use salted butter if you wish.
Be sure the butter is softened before you mix it. You can set it out on the counter or soften it in the microwave on low power.
The cocoa powder and powdered sugar do not need to be sifted prior to mixing.