Root Beer Float with muffin

1 (18 1/4 ounce) package white cake mix 1 1/4 cups root beer 1/4 cup vegetable oil 2 large eggs 2 teaspoons vanilla extract 1 (16 ounce) can vanilla frosting[...]

3/4 cup butter 3 eggs 2 1/2 cups all-purpose flour 2 1/2 teaspoons baking powder 1/2 teaspoon salt 1 1/2 cups sugar 1 tablespoon root beer extract 1/2[...]