3 (100g) egg whites
2/3 c (130g) sugar
10 Tbsp (142g) butter, cubed and at room temp
1 tsp vanilla
4 oz (113g) cream cheese, cubed and at room temp
In the base of a double boiler, bring water to a simmer. In the top bowl (or just a bowl to place over a pan of simmering water) combine the egg whites and sugar and whisk constantly until the mixture is hot to the touch (160F on a candy thermometer if you want to be completely accurate), about five minutes depending on the original temperature of the eggs. When the mixture has warmed sufficiently, pour it into the bowl of your stand mixer and whip on high speed until the bottom of the bowl is room temperature, ten to fifteen minutes. When the bowl is no longer warm, turn the mixer to medium-low and begin adding the butter piece by piece. Let each chunk incorporate completely before adding the next and don't rush the process!
After all of the butter has been added, add the vanilla and turn the speed back to high. Whip for about 3 minutes or until the buttercream comes together. Turn the speed down to low and add the cream cheese, then mix on medium to fully incorporate.