Preheat the oven to 375°F. In a large bowl, whisk together the flour, cornmeal, baking powder, salt, and sugar. In a separate bowl, whisk together the buttermilk and egg yolks. Set aside.

Melt the lard in a 10-inch cast-iron skillet over medium heat. Add the corn kernels and quickly sauté until fragrant and the corn just begins to soften, about 3 minutes. Turn off the heat and allow the corn to cool slightly in the pan.

Add the buttermilk and yolk mixture to the dry ingredients and stir just until the batter comes together. Remove the corn from the cast-iron skillet with a slotted spoon and gently mix it into the batter. Reserve the skillet with the remaining lard for baking the cornbread.

In a medium bowl, whip the egg whites with an electric mixer until medium-stiff peaks form. Fold them into the cornbread batter with a rubber spatula. Pour the batter into the reserved cast-iron skillet and bake about 25 minutes, until set and golden.