From The Magnolia Bakery Cookbook
Makes enough for one 2-3 layer 9-inch cake or way more than enough for 24 cupcakes
1 cup (2 sticks) butter, unsalted and very soft
5-8 cups powdered sugar
1/2 cup milk
2 teaspoons vanilla extract
Put softened butter in a stand mixer. Add 4 cups powdered sugar. Then pour in 1/2 cup milk and 2 teaspoons vanilla. Beat until it becomes creamy - make sure all the lumps are gone. Add in sugar 1 cup at a time, mixing thoroughly each time until it is to the correct sweetness and thickness. If you want it thicker, keep mixing it.
Store icing at room temperature (it will set if chilled) for up to three days.