Preheat oven to 350 degrees F. Generously butter a 9 x 9-inch square baking pan. If desired, line bottom of pan with a buttered square of parchment.

For best results seeding Concord grapes, start with fresh, firm grapes and use a sharp knife to cut in half; flick seeds out with the knife tip. Don't worry if skins slip from the flesh or if seeds needs to be squeezed out.

In a small mixing bowl, whisk to combine flour, cornmeal, baking powder, and sea salt. In the bowl of a stand mixer fitted with the paddle attachment or in a large mixing bowl, combine eggs and 3/4 cup sugar, and beat until thick and frothy, about 2 minutes. Add buttermilk and zest, and beat 1 minute more. Scrape down sides, switch mixer to medium speed, and drizzle in olive oil. Switch mixer to low, add half the flour mixture; as soon as it's incorporated, add the second half.

Strain any accumulated juices from the grapes, and fold all but 1/4 cup into the batter.

Spoon batter into prepared baking pan and bake 5 minutes. 

Working quickly, remove pan from oven, scatter remaining grapes over top, pressing gently into the batter. Finish with 2 tablespoons sugar. Slide back into oven and bake until cake is a rich golden hue and the cake begins to pull away from the sides, 25 - 30 minutes more.

Cool 30 minutes, slice, and serve. Cake is best day-of, but will keep well covered at room temperature for a day.