In bowl, blend confectioners' sugar, butter, milk and vanilla until smooth. Once cupcakes have cooled, spread with frosting, about 1-1/2 tablespoons for each. Make a small indent in center of frosting with a spoon. Spoon scant 1/2 teaspoon cooled strawberry mixture into indent on top of each cupcake. Chill 15 minutes, or until set.