Nectarine, Cherry Tomato + Cornbread Panzanella

Make the cornbread: Preheat the oven to 375 degrees. In a cast iron skillet, melt the butter over medium heat, then turn the burner off. In a mixing bowl, whisk together all dry ingredients. In a separate bowl, whisk together the egg and buttermilk, then combine with the dry ingredients. If it seems too dry, add a bit more buttermilk. Pour the batter into the cast iron skillet and spread it evenly across the pan. Bake for 30 minutes, or until the edges have browned. If you stick a fork in it, it should come out clean. Let cool, then transfer to a cutting board and cut the cornbread into small cubes. Crank the heat on the oven back up to 400 degrees, and transfer the cubed cornbread to a large baking pan. Bake for 10 minutes, or until the bread is firmer, like a crouton. (Note: the cornbread can be made a day ahead of time, but I found Bittman's recipe to yield a bread that was dense and firm, yet soft enough to soak up the fruit juice.)
While the cornbread is baking, make the salad: Add tomatoes, nectarines, basil, mozzarella, olive oil, balsamic, and sea salt to a large mixing bowl. Toss until combined, then let sit for 10-15 minutes to let the fruit get juicy.
Once they've cooled, add the cornbread croutons to the mixing bowl. Toss until combined, then let the whole thing sit for another 10-15 minutes, so the cornbread can soak up the juices.