Make sure butter is at room temperature before beginning recipe.
Place butter and vanilla into a mixing bowl or stand mixer; beat at MEDIUM speed for about 3 minutes. You want mixture to become light and fluffy.
Combine 2 lb. bag confectioner's sugar, cocoa and salt in a separate bowl and stir well to thoroughly combine ingredients.
With mixer off, add one cup of cocoa mixture, then turn to LOW and beginning mixing. Slowly add in remaining cocoa mixture, about 1/4 cup at a time,
With mixer still on LOW speed, slowly pour in the whole milk. Stop mixer to scrape sides of bowl as needed. Turn mixer to MEDIUM speed and let blend for 2-3 minutes. Add more milk or confectioner's sugar, as needed.
This is plenty of frosting for a 2 or 3 layer cake.