1 C sugar
2 oz water
4 egg yolks
1/2 t salt
1 lb butter, softened
1/2 lb shortening, softened
Dissolve the sugar in water and cook until a candy thermometer reads 240.
Beat the eggs, yolks, and salt with an electric mixer until light & fluffy.
Continue whipping while you pour the sugar syrup over the eggs, pouring it in a steady stream between the beaters and the side of bowl. Continue to whip on high until cool, then switch to low and mix in the butter and shortening.