Preheat the oven to 350 degrees F. Spray a 12-cup muffin tin with cooking spray.
Place the cauliflower in a large microwave-safe bowl and cover with plastic wrap. Microwave until warm and tender, 3 to 4 minutes. Let cool for 10 minutes, then transfer to a clean kitchen towel and squeeze tightly to release as much moisture as possible.
Wipe the bowl clean and add back the cauliflower. Add the cornmeal, butter, sugar, baking powder, egg yolks, 1 cup of the Cheddar, the diced jalapeno, 1 teaspoon salt and a few grinds of black pepper and stir until well combined.
Beat the egg whites in a medium bowl with an electric mixer on medium-high speed until stiff peaks form, 3 to 4 minutes. Fold half of the whites into the cauliflower mixture until combined. Repeat with the remaining egg whites.
Pour a scant 1/2 cup batter into each muffin cup and sprinkle with the remaining 1/2 cup Cheddar. Place a few jalapeno slices on top of the batter in each cup. Bake until a toothpick inserted into the center of the muffins comes out clean and the tops are golden brown, about 20 minutes. Let cool for 10 minutes, then remove the muffins to a rack to cool completely. Serve at room temperature.
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