Line 8-inch square baking pan with foil, with ends of foil extending over sides. Spray with cooking spray. Microwave 2 oz. chocolate and butter in medium microwaveable bowl on HIGH 1 min. 30 sec. or until butter is melted. Stir until chocolate is completely melted. Stir in 1/2 cup sugar. Blend in 1 egg. Add flour; mix well. Spread onto bottom of prepared pan.
Microwave remaining chocolate and cream in microwaveable bowl 1 min. 30 sec.; stir until chocolate is completely melted. Beat remaining sugar and eggs in large bowl with mixer on high speed 1 min. or until thick and lemon colored. Add cream mixture; mix well. Pour over batter in pan.
Bake 35 min. or until topping is set. Cool in pan. Run knife around sides of pan to loosen dessert; use foil handles to remove from pan before cutting to serve. Keep refrigerated.