Preheat oven to 350°.
Coat an 8-inch square pan with cooking spray, and dust the pan with 2 teaspoons flour.
Place butter in a large microwave-safe bowl. Cover and microwave at high 1 minute or until butter melts. Add cocoa, stirring well with a whisk. Add brown sugar, stirring well until mixture pulls away from sides of bowl. Add eggs, 1 at a time, stirring with a whisk until smooth. Stir in baking soda, vanilla, and salt. Lightly spoon 3/4 cup flour into dry measuring cups; level with a knife. Gradually add flour to bowl, stirring just until blended (do not overstir). Combine hot water and espresso. Add to flour mixture, stirring just until blended.
Pour batter into prepared pan. Bake at 350° for 25 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes on a wire rack. Sprinkle with powdered sugar. Cut into squares; serve warm or at room temperature.