Preheat oven to 400°F (200°C). Grease a 12-cup muffin tin or line with paper liners.
In a large bowl, mix together cornmeal, flour, sugar, baking powder, and salt.
In another bowl, combine milk, vegetable oil, and beaten egg. Add to dry ingredients and stir until just combined.
Fold in chopped jalapeños. Spoon batter into prepared muffin tin, filling each cup about ⅔ full.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Let cool in the pan for a few minutes before transferring to a wire rack to cool completely.