Preheat oven to 350°F. Butter a 9-by-13-inch baking pan.
In a medium bowl, whisk flour, baking powder, baking soda and salt.
In a large bowl, using an electric mixer on high speed, beat butter and sugar until light, about 3 minutes. Add eggs, 1 at a time, beating well after each and scraping down sides of bowl. Reduce speed to low; beat in chocolate and vanilla.
Add flour mixture in 3 parts, alternating with buttermilk. Fold in zucchini (batter will be stiff). Spread evenly in pan; bake until top springs back slightly when touched in center, 35 to 40 minutes. Let cool in pan for 10 minutes. Turn out onto a wire rack. Dust with confectioners' sugar just before serving, if desired.