Position a rack in the middle of oven and preheat to 350°. Spray with cooking spray or butter a 9 x 5 x 3" loaf pan, mini loaf pans or a muffin tin, depending on which you choose to use.Melt 1/2 cup butter in a small saucepan over medium-high heat. Continue cooking until butter solids at bottom of pan turn golden brown, about 3 minutes. Scrape butter into a medium bowl. Set aside and let cool. Whisk in eggs and buttermilk.
Trim zucchini ends. Thinly slice five 1/8" rounds from 1 end of zucchini, if making the large loaf, and reserve for garnish. (You will need more zucchini slices if making the mini loaves or muffins. I used one small zucchini sliced in 1/8 inch slices.) Coarsely grate remaining zucchini. Add to bowl with butter mixture and stir until well blended. Sift both flours, sugar, baking powder, salt, and baking soda into a large bowl. Whisk in cornmeal. Add zucchini mixture; fold just to blend (mixture will be very thick). Transfer batter to prepared pan and smooth top. Place reserved zucchini slices atop batter down center in a single layer.
Bake bread until golden and a tester inserted into center comes out clean, 55-65 minutes. *The mini loaves take about 35 minutes and the muffins take about 25 minutes to cook. Let cool in pan 10 minutes. Remove from pan; let cool completely on a wire rack. Can be made 1 day ahead. Store airtight at room temperature.
*The bread is delicious as is but next time I think I'll cut back on the sugar just a bit, for a slightly more savory bread.