For a 9 X 13 pan:
Cream together 4 Tbsp. butter and 6 oz. cream cheese. Gradually add 1/2 C. sugar; cream well after each addition. Blend in 2 eggs, 2 Tbsp. flour, and 1 tsp. vanilla. Set aside. After mixing the brownie batter, spread about half of it in a 9 X 13 pan. Spread the cheese mixture over the top; spoon on remaining brownie batter. Zigzag through batters with a spatula to marble. Bake according to brownie instructions, possibly adding more time.