Preheat oven to 350°.
Press zucchini between several layers of paper towels; cover with additional paper towels. Let stand 5 minutes, pressing down occasionally.
Combine sugar and eggs in a large bowl; beat well with a wooden spoon. Stir in zucchini, buttermilk, butter, and vanilla.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 5 ingredients in a medium bowl. Gradually add flour mixture to zucchini mixture, stirring just until moist.
Spread batter into a 9 x 5-inch loaf pan coated with cooking spray. Bake at 350° for 53 to 55 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack 10 minutes. Remove loaf from pan. Cool completely on wire rack.