Prepare pasta according to package directions, but cook for 2 minutes less than manufacturer's directions suggest. Drain pasta. Mix in 2 tablespoons butter with pasta and set aside.
Place remaining 6 tablespoons butter, 2 cups of buttermilk, and ½ cup milk in saucepan and heat gently until butter is just melted. Using a wire whisk, mix in flour, salt, and pepper while stirring. Adjust to medium heat and seasonings. Continue heating and stirring until mixture thickens (about 5 minutes). Remove from heat and add 3 cups medium cheddar cheese and 1 cup white medium cheddar Cheese. Mix until cheese melts slightly. Add pasta. Pour into greased 9”x13" baking pan.
In small bowl mix biscuit mix, 1/2 cup buttermilk, and 1/2 cup medium cheddar cheese. Drop biscuit mix by small teaspoons on top of macaroni. Bake until golden brown on top - 15-20 minutes.