For this recipe I set aside a small amount of buttercream frosting at the end and added cocoa powder to make it chocolate, and left the rest a vanilla buttercream. But you are free to follow this or not.
While the cake is baking, wash and prep your mixer and add in your room temperature sticks of butter. These need to be very room temperature or else it will take some effort to get the butter fluffy.
Add in the confectioners sugar and mix on low speed for 3-4 minutes.
Add in 1-2 tbsp of your choice of liquid (milk, cream or plant-based milk) and vanilla. Beat until thoroughly mixed and stiff.
If you feel you need more liquid, add in more by 1-2 tbsp.
If you need to make more buttercream frosting, continue with 1 part butter with 2 parts confectioners sugar depending on how much you need.
To make chocolate buttercream frosting: simply add 1/4-3/4 cup cocoa powder, depending on the amount of buttercream you use.