1 pound unsalted butter, soft and cut into one inch pieces
1 cup and 1/2 cup sugar
1/3 cup stout (such as Guinness)
5 egg whites
1/4 tsp cream of tartar
In the bowl of a stand mixer, place egg whites, with cream of tartar and 1/2 cup sugar nearby.
Heat 1 cup sugar and 1/3 cup stout in a small saucepan.
Heat over medium high heat until sugar is dissolved.
When sugar reaches 230F on a candy thermometer, turn mixer on medium high.
When egg whites are frothy, add cream of tartar. Gradually add the 1/2 cup sugar.
When egg whites begin to form soft peaks, turn the mixer down to medium low and begin to drizzle in the boiling sugar mixture (which should be at approximately 245-250F, firm ball stage).
When all of the hot sugar is added, turn the mixer up to medium high and beat until the bowl is no longer warm to the touch.
Add the butter one lump at a time and continue beating until mixture is smooth and fluffy, approximately 12-20 minutes.
It will look rather like ricotta cheese for a while - just keep beating! Top butterbeer cake with buttercream "head".
Keep at room temperature - do not refrigerate.