While the cookie cups are cooling, make the vanilla buttercream frosting. Using a stand mixer or handheld mixer, beat the butter on medium speed until creamy, about 2-3 minutes. With the mixer on low, add 4 cups confectioners' sugar, the heavy cream, and vanilla extract. Turn up the speed to high and beat for 2-3 minutes or until frosting is smooth. If the frosting is too thin, add 1/2-1 cup more confectioners' sugar and beat again. If the frosting is too thick, you can thin it out with a little extra heavy cream. Only add 1 tablespoon at a time.