Blackberry Buttercream

Name: Blackberry Buttercream
Yield: 15 cups
Ingredients: 6 large eggs, separated; 1 1/2 cups sugar; 2 cups whole milk; 1 teaspoon pure vanilla extract; 6 cups (12 sticks) unsalted butter, softened; 1/2 cup (about 3 cups whole berries) blackberry puree