1. Using a hand mixer, cream the butter on medium speed until light and fluffy, 3 to 4 minutes. Add the vanilla and mix for 1 minute. Scrape down the bottom and sides of the bowl.
2. With the mixer running on low speed, gradually add the powdered sugar, 1 cup (125g/4.4oz) at a time. Scrape down the bottom and sides of the bowl again. When you have added the third cup of sugar, the mixture will start to thicken and appear dry. Add 3 tablespoons of milk, then add the rest of the sugar and add more milk if the buttercream is too stiff. The desired consistency is creamy and smooth. Scrape down the bottom and sides of the bowl. The great thing about buttercream is that you can adjust the quantities of sugar and milk to achieve the consistency you desire.
3. Continue to beat the icing on low speed for 1 to 2 minutes longer, until smooth and creamy. Do not mix on medium or high, as it will add too much air and be too fluffy. If desired, add a few drops of food coloring, one drop at a time, and mix thoroughly to get the right color.
4. Store the icing at room temperature in an airtight container for up to 3 days. 

Makes: 4 cups (enough for one 9-inch or 24 cupcakes)