Preheat the Oven:

Preheat your oven to 400°F (200°C) and line a muffin tin with paper liners or grease it with cooking spray.

Mix Dry Ingredients:

In a large bowl, whisk together the cornmeal, flour, sugar, baking powder, and salt.

Combine Wet Ingredients:

In another bowl, whisk the buttermilk, melted butter, eggs, and vegetable oil until smooth.

Mix the Batter:

Pour the wet ingredients into the dry ingredients and stir gently until just combined. If adding cheese or jalapeños, fold them in now. Be careful not to overmix; the batter should be slightly lumpy.

Fill Muffin Tin:

Spoon the batter into the prepared muffin tin, filling each cup about 3/4 full.

Bake:

Bake for 15–18 minutes, or until the tops are golden and a toothpick inserted into the center comes out clean.

Cool and Serve:

Let the muffins cool in the tin for 5 minutes before transferring to a wire rack. Serve warm or at room temperature.