Preheat the oven to 450°F. Put a 9-inch cast-iron skillet in the oven to preheat for at least 10 minutes.
Run the bacon through a meat grinder or very finely mince it. Put the bacon in a skillet large enough to hold it in one layer and cook over medium-low heat, stirring frequently so that it doesn’t burn, until the fat is rendered and the bits of bacon are crispy, 4 to 5 minutes. Transfer the bits of bacon to a paper towel to drain, reserving the fat. You need 5 tablespoons bacon fat for this recipe.
Combine the cornmeal, salt, baking soda, baking powder, and bits of bacon in a medium bowl. Reserve 1 tablespoon of the bacon fat and combine the remaining 4 tablespoons fat, the buttermilk, and egg in a small bowl. Stir the wet ingredients into the dry ingredients just to combine; do not overmix.
Move the skillet from the oven to the stove, placing it over high heat. Add the reserved tablespoon of bacon fat and swirl to coat the skillet. Pour in the batter, distributing it evenly. It should sizzle.
Bake the cornbread for about 20 minutes, until a toothpick inserted in the center comes out clean. Serve warm from the skillet.
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