Preheat oven to 325°.
Place sugar and eggs in a large bowl; beat with a mixer at high speed until thick and pale (about 5 minutes). Combine hot water and coffee granules, stirring until coffee granules dissolve. Add coffee mixture, butter, and vanilla extract to sugar mixture; beat at low speed until combined.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, cocoa, and salt, stirring with a whisk. Gradually add flour mixture to sugar mixture, stirring just until moist (batter will be thick).
Spread batter into an 8-inch square baking pan coated with cooking spray. Bake at 325° for 23 minutes or until brownies spring back when touched lightly in center. Cool in pan on a wire rack. Garnish brownies with powdered sugar, if desired.