HEAT oven to 375º F. Spray bottom of 8-inch square cake pan with no-stick cooking spray. Stir together flour, sugar and butter in medium bowl until blended. (Mixture will be like soft dough). Set aside.
Blend muffin mix, milk and sour cream in a large bowl. Stir until dry ingredients are moistened. Spread into pan. Crumble topping over batter. Bake for 35 to 40 minutes or until golden brown.