Preheat oven to 350 degrees F (180 degrees C). Grease a 13x9-inch pan.

Microwave chocolate and butter in large bowl at HIGH for 2 minutes or until butter is melted. Alternately, melt butter and chocolate in a double boiler over simmering water.

Remove bowl from microwave and stir until chocolate is melted. Stir in sugar. Mix in eggs and vanilla. Stir in flour and nuts.

Spread batter into prepared pan. Bake for 35 minutes (do not overbake).