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Cutting the butter will make it easier to add it to your recipe later on. It will also make it easier to mix. Place a heat-proof bowl over a pot filled with some water. The bottom of the bowl should not be touching the water. Set the pot on the stove and set the heat to medium. If you cannot tell whether or not the sugar has dissolved, you can rub the mixture between your fingers. If the mixture feels grainy, the sugar has not finished dissolving. This pasteurizes the egg whites and kills any salmonella-causing bacteria. Be sure to use an instant-read thermometer. At this point, you can use an electric mixer or transfer it to a food processor with a whisk attachment. Use a medium-high speed. After about ten minutes, the egg whites will start to form peaks and turn stiff and foamy. Turn the speed down to medium low, and add in the vanilla and butter. If you do not like vanilla, you can substitute with 1 teaspoon of another extract, such as almond. Click here for more options. If you prefer your buttercream to be less sweet, then add a pinch of salt. Once the buttercream has achieved a smooth consistency, you can spread the buttercream over a cake or some cupcakes. You can also put it into a plastic Ziploc bag or an airtight container and store it in the refrigerator for up to two weeks.

Summary:
Cut the butter into cubes and set it aside. Assemble a double-boiler and let the water begin to simmer. Add the egg whites and sugar and mix until the sugar dissolves. Let the mixture heat until it reaches 160°F (72°C). Remove the mixture from the double-boiler and continue mixing it until the egg whites become stiff. Reduce the speed and add the vanilla and butter. Store or serve the buttercream.