4 large egg whites
1 1/4 cups sugar
1 1/2 cups (3 sticks) unsalted butter, room temperature, cut into tablespoons
1 teaspoon pure vanilla extract
1 1/2 cups (12 ounces) strawberry jam, pureed in a food processor
In the heatproof bowl of an electric mixer set over a saucepan of simmering water, combine egg whites and sugar. Cook, whisking constantly, until sugar has dissolved and mixture is warm to the touch (about 160°).
Attach the bowl to the mixer fitted with the whisk attachment. Beat the egg-white mixture on high speed until it holds stiff (but not dry) peaks. Continue beating until the mixture is fluffy and cooled, about 6 minutes.
Switch to the paddle attachment. With mixer on medium-low speed, add butter several tablespoons at a time, beating well after each addition. (If frosting appears to separate after all the butter has been added, beat on medium-high speed until smooth again, 3 to 5 minutes more.) Beat in vanilla. Beat on lowest speed to eliminate any air bubbles, about 2 minutes. Stir in strawberry jam with a rubber spatula until frosting is smooth.
Makes 5 cups