1 cup wheat flour
1/2 cup finely ground cornmeal
1 1/2 teaspoons baking powder
1/2 teaspoon fine salt
8 tablespoons (1 stick) unsalted butter, at room temperature
1 cup granulated sugar
2 large eggs, at room temperature
1/2 cup plain organic yogurt
1 teaspoon vanilla extract
Finely grated zest and juice of 1 medium lemon
1 1/2 cups fresh blueberries or raspberries
Arrange a rack in the middle of the oven and heat to 350°F. Coat a 12-inch cast iron skillet with cooking spray; set aside.
Whisk the flour, cornmeal, baking powder, and salt together in a medium bowl. Place the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment and mix on medium speed until light and fluffy, about 3 minutes. (Alternatively, you can use a hand mixer on medium-high speed instead.)
Add the eggs one at a time and beat until combined. Stop the mixer and scrape down the sides of the bowl and the paddle with a rubber spatula. Add the yogurt, vanilla, juice and zest; return the mixer to medium speed; and mix until combined, about 1 minute (the mixture will look curdled). Reduce the mixer speed to low, gradually add the flour mixture, and mix until just combined.
Remove the bowl from the mixer, add the blueberries, and gently fold in by hand with the rubber spatula. Transfer the batter to the skillet and spread into an even layer.
Bake until golden-brown around the edges and a tester inserted into the center of the cake comes out clean, about 30 minutes. Move the skillet to a wire rack and let cool for at least 20 minutes. Run a knife around the edge to loosen the cake, cut into wedges, and serve.
The cake can be kept wrapped in plastic wrap at room temperature for up to 3 days.