Serves: Makes about 5 cups, enough for a 2-layer, (8- or 9-inch) cake with piped borders, or 24 cupcakes
- 4 large egg whites
- 1½ cups (10½ ounces) sugar
- 1 pound unsalted butter, very soft, but not melted
- 2 teaspoons pure vanilla extract
- Place the egg whites and sugar in the bowl of the mixer (or the medium bowl, if using a hand mixer) and hand-whisk to blend. Bring 2 inches of water to a gentle boil in the medium saucepan. Place the bowl over the simmering water and heat, whisking constantly so that the whites do not scramble, until the mixture reaches 160°F on the thermometer. Be sure to remove the bowl from the heat when taking the temperature so the eggs don't overheat while you set down the whisk and use the thermometer. If the temperature is not hot enough, rinse the thermometer under very hot water (so you don't contaminate the whites with any bacteria left on the thermometer when you test again), dry and set aside, then put the whites and sugar back over the heat.
- Once the correct temperature is reached, immediately remove the mixture from the heat and whip with the mixer on high speed until it has cooled to room temperature, is light and billowing, and resembles marshmallow fluff. If the mixture is warm, it will melt the butter you'll be adding next, so touch the meringue and make sure it has cooled to room temperature before continuing.
- With the mixer on medium-high speed, add the soft butter a couple of tablespoons at a time, allowing each addition to blend in fully before adding the next. Halfway through the butter, stop the mixer and use the spatula to scrape down the sides of the bowl thoroughly. Continue until all the butter has been added and the mixture resembles velvety mayonnaise. Add the vanilla extract and blend well. Note: There is nearly always a point in this process when you think the buttercream is ruined. It will look broken and awful. Just continue adding the butter and beating and all will be fine in the end. Use the buttercream immediately or refrigerate or freeze until needed.
Storing: Buttercream may be stored in an airtight container in the refrigerator for up to 5 days.