HEAT oven to 350ºF.
MICROWAVE chocolate and butter in microwaveable bowl on HIGH 1 min. or until butter is melted. Stir until chocolate is completely melted and mixture is well blended.
WHISK eggs and sugar in medium bowl until blended. Stir in coffee, zest and chocolate mixture. Add flour; mix well. Stir in nuts. Pour into foil-lined 8x4-inch pan.
BAKE 30 to 35 min. or until toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Cool in pan 10 min. Remove to wire rack; cool completely.