Coat an 8-inch square baking pan with cooking spray and dust with 2 tsp. flour. Combine brown sugar, walnuts, cinnamon and 1 tsp. espresso powder in a small bowl.
Place the butter in a large microwave-safe bowl. Cover and microwave at HIGH for 1 minute or until butter melts. Add granulated sugar, stirring with a whisk. Add eggs; stir well. Stir in yogurt, vanilla, baking soda and salt. Lightly spoon 1 1/3 cups flour into a dry measuring cup, level with a knife. Add flour and 2 tsp. espresso powder, stirring just until blended (do not overmix).
Spread half of the batter into the prepared pan and sprinkle with half of the brown sugar mixture. Carefully spread remaining batter over brown sugar mixture and sprinkle with remaining brown sugar mixture.
Bake at 350 degrees for 25 minutes or until a wooden toothpick, inserted in center, comes out clean. Cool for 10 minutes in pan on a wire rack. Cut into squares.