For the topping:
6 tablespoons (85 grams) unsalted butter, plus more for coating the pan
2/3 cup (135 grams) granulated sugar
4 small nectarines (about 450 grams), halved and pitted
3 cups blackberries (340 grams)
For the cake:
2 cups (225 grams) cake flour
1/2 cup (70 grams) cornmeal
2 teaspoons baking powder
1/2 teaspoon baking soda
1 teaspoon fine salt
3/4 cup (170 grams) unsalted butter, at room temperature
1 1/4 cups (250 grams) granulated sugar
3 large eggs
1 1/4 cups (300 milliliters) well-shaken buttermilk
2 teaspoons vanilla extract
Whipped cream or vanilla ice cream, for serving (optional)
Arrange a rack in the center of the oven and heat to 350°F. Line a 9x13-inch baking pan with parchment paper. Butter the pan and the paper.
Make the topping: Combine the 6 tablespoons butter, sugar, and salt in a medium saucepan over medium heat. Cook, whisking occasionally, until the butter and sugar are melted. Pour the mixture into the prepared pan and tilt the pan to spread the mixture evenly.
Cut the nectarines into 1/2-inch slices. Place them cut-side down in the bottom of the pan. Sprinkle the blackberries over the top. The pan should be completely lined with a single layer of fruit; set aside.
Make the cake: Whisk the cake flour, cornmeal, baking powder, baking soda, and salt together in a medium bowl.
In a stand mixer fitted with the paddle attachment or with an electric hand mixer, cream the butter and sugar together until light and fluffy, 3 to 5 minutes. Add the eggs one at a time, mixing for 30 seconds in between each egg. Stop the mixer periodically to scrape down the sides of the bottom of the bowl to ensure even mixing.
Add the vanilla to the buttermilk.
On low speed, alternate adding the flour mixture and buttermilk to the butter-sugar mixture in 3 additions until just combined. Finish mixing the batter by hand with a rubber spatula, making sure to scrape down to the bottom of the bowl to ensure the batter is evenly mixed.
Spread the cake batter evenly over the fruit, and then firmly tap the pan on a countertop to help the batter settle between the fruit and release large air pockets.
Bake the cake, rotating halfway through the baking time, until golden, puffed, and a cake tester inserted into the center comes out clean, 40 to 50 minutes total.
Let the cake cool on a wire rack for 15 minutes. Carefully invert onto a serving board or platter. Remove the parchment paper from the top. Serve warm with ice cream or whipped cream on the side.