Coat an 8-inch square baking pan with cooking spray, and dust with 2 teaspoons flour.
Combine brown sugar, walnuts, cinnamon, and 1 teaspoon espresso granules 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 dry measuring cups; level with a knife. Add flour and 2 teaspoons espresso powder, stirring just until blended (do not overstir).
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° for 25 minutes or until a wooden pick inserted in center comes out clean. Cool for 10 minutes in pan on a wire rack. Cut into squares.