Heat oven to 350
Line 13x9-inch pan with nonstick foil.
Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted.
Stir in sugar.
Add eggs and vanilla.
Add flour.
Pour in pan.
Bake 30 to 35 min. or until toothpick inserted in center comes out with fudgy crumbs.