Preheat oven to 350°.
Combine 1/4 cup boiling water and espresso in a medium bowl. Add chocolate chips, stirring until chocolate melts; cool slightly. Stir in butter, vanilla, egg, and egg white.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 4 ingredients (through salt), stirring with a whisk. Add coffee mixture to flour mixture, stirring just until moist. Spoon batter into a 9-inch square baking pan coated with cooking spray. Bake at 350° for 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack. Sprinkle with powdered sugar, if desired.