Preheat the oven to 325 degrees F. Lightly spray 1 large baking sheet with vegetable spray oil. In a food processor fitted with the metal blade, combine the butter and sugar and process until blended. Add the egg whites and process until smooth. Add the flour, baking powder and salt and pulse just until incorporated. Transfer to a bowl and add the cocoa and vanilla extract. Stir in the nuts until incorporated. Divide the dough into 3 flattish logs about 12 inches long and 1 1/2 inches wide. Lay the logs on the baking sheet about 2 1/2 inches apart and bake for 25 to 30 minutes, until lightly browned. The logs will flatten slightly as they bake. Transfer the logs to a cutting board and cool for 10 to 15 minutes. Reduce the oven temp to 275 degrees F. With a sharp knife, cut the logs diagonally into 1/2 inch thick slices. Arrange the slices, standing upright, on the baking sheets and bake 15 to 25 minutes, or until the cut surfaces dry out slightly. Cool on wire racks to room temperature. Serve immediately or store in an airtight container.