Mocha Brownies

Preheat oven to 375°F. Spray 13x9x2-inch metal baking pan with nonstick spray. Line with waxed paper, extending over 2 sides of pan. Spray paper. Stir chocolate and butter in heavy small saucepan over low heat until melted. Cool to lukewarm.

Using electric mixer, beat 2 cups sugar, eggs and vanilla in medium bowl until fluffy, about 5 minutes.

Add chocolate mixture; beat just until blended.

Add flour and next 3 ingredients; beat just until blended. Stir in sour cream and nuts.

Transfer batter to pan.

Bake until tester inserted into center comes out with moist crumbs attached, about 35 minutes.

Transfer pan to rack; cool completely.

Cut into squares. Top with powdered sugar.