Strawberry Rhubarb Cupcakes

Ingredients

• 2 cups rhubarb, cut into small pieces
• 1 cup strawberries, stems removed and roughly chopped
• 7 tablespoons granulated sugar
• 2 teaspoons pure vanilla extract
• 5 large egg whites, at room temperature
• 1/2 cup whole milk, at room temperature
• 1-1/2 teaspoons pure vanilla extract
• 2-1/2 cups cake flour
• 1-3/4 cups granulated sugar
• 1 tablespoon + 1 teaspoon baking powder
• 3/4 teaspoon salt
• 12 tablespoons unsalted butter, at room temperature and cut into cubes
• 1/4 cup strawberry puree made from fresh strawberries*, room temperature
• 3 sticks (24 tablespoons) unsalted butter, room temperature
• 3 cups powdered sugar
• 1/2 teaspoon pure vanilla extract
• 5 tablespoons strawberry puree, room temperature