1. Heat oven to 350 degrees F. Coat several large baking sheets with cooking spray.
2. Microwave chocolates and butter, covered, in microwave-safe bowl at full power 2 minutes. Uncover; stir until melted and smooth. Set aside to cool.
3. With mixer at high speed, beat sugar, eggs, espresso powder and vanilla in large bowl until light and fluffy.
4. Mix flour and baking powder in small bowl. Whisk flour mixture into cooled chocolate mixture. Whisk chocolate mixture into egg mixture. Stir in walnuts. Drop mixture by slightly rounded 1-1/2 tablespoonfuls, about 2 inches apart, on prepared sheets.
5. Bake in 350 degrees F oven 9 minutes, until puffed, slightly crisped around edges. Cool on baking sheet on rack 1 minute. Remove with spatula to rack to cool.