Preheat oven to 350°F. Lightly butter 8-inch square baking pan. Stir 1/2 cup butter and chocolate in heavy medium saucepan over medium-low heat until melted and smooth. Remove from heat; mix in coffee powder. Add sugar, flour, eggs and vanilla and blend well. Mix in walnuts. Spoon batter into prepared pan. Bake brownies until tester inserted into center comes out with a few moist crumbs attached, about 20 minutes (center should be slightly underbaked). Cool in pan on rack 30 minutes. Cut into large squares and serve.