1 pound (4 sticks) unsalted butter, cut into tiny cubes at room temperature
1 3/4 cups sugar
1/4 cup water
5 egg whites
1/4 teaspoon cream of tartar
1 teaspoon vanilla
1. In a bowl of a stand mixer, place egg whites with cream of tartar and 1/4 cup of sugar nearby.
2. Heat 1 1/2 cup sugar and 1/4 cup water in a small saucepan. Heat over high medium heat until sugar is dissolved
3. Meanwhile, whip the whites at high speed until soft peaks from approximately 5 minutes. When the egg whites are frothy, add the cream of tartar. Gradually add the 1/4 cup remaining sugar.
4.When the sugar reaches 240 degrees F, with the motor running, pour it into the egg whites, very slowly, in a thin stream, to avoid cooking the eggs. Raise the speed to high, and continue to whip the mixture has cooled to room temperature between 10-15 minutes.
5. Stopping the motor between additions, add the butter one lump at a time and continue beating until mixture smooth and fluffy, approximately 12-20 minutes.
Note: It will look like ricotta cheese for a while..just keep beating.
6. Once it is smooth, stir in flavorings if desired.