6 egg yolks
1/2 cup water
1 cup sugar
1 pound butter at warm room temperature
3/4 cup seedless mixed berry jam
In a bowl of a stand mixer whip yolks until thick and light. Heat sugar and 1/2 cup water over medium heat until the syrup reaches 238 degrees. Immediately pour into a Pyrex measuring cup or small metal bowl to stop cooking. Stop the mixer. Add the sugar syrup and then resume whipping until the outside of the bowl is cool.
Remove whisk and add paddle attachment. Add butter in small chunks, beating until frosting is silky and smooth. Add berry jam.