At first, your frosting will be thick. Adding in vanilla extract and a bit of milk (heavy whipping cream or milk alternative) will thin it out a bit. That’s why you only want to add in about ½ Tbsp at a time so you don’t make it too thin. But this will help. Honestly, your frosting with just butter and powdered sugar might look like dough in the beginning, don’t worry. Adding in the liquid will thin it out, just keep whipping!