Preheat your oven to 190°C (375°F). Grease a 23cm (9-inch) square baking pan or an 23cm (9-inch) round cake pan with vegetable oil. Alternatively, you can line it with parchment paper for easier removal.

In a large mixing bowl, whisk together the sifted all-purpose flour (250g or 2 cups), yellow cornmeal (200g or 1 1/2 cups), granulated sugar (100g or 1/2 cup), baking powder (15g or 1 tbsp), and salt (5g or 1 tsp). Ensure all dry ingredients are well combined to distribute the leavening agent evenly.

Add the finely diced jalapeño peppers (approx. 75g or 2.6 oz) and the shredded cheddar cheese (150g or 1 1/2 cups) to the dry ingredients. If using corn kernels (170g or 1 cup), add them now as well. Toss everything gently to coat the jalapeños and corn with the dry mixture. This helps prevent them from sinking to the bottom during baking.

In a separate medium bowl, whisk together the room temperature large eggs (2), melted and slightly cooled unsalted butter (115g or 1/2 cup), and room temperature buttermilk (240ml or 1 cup) until well combined and smooth.

Pour the wet ingredients into the bowl with the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few small lumps are perfectly fine. Overmixing can lead to a tough cornbread.

Pour the batter into the prepared baking pan, spreading it evenly with the spatula. The batter will be thick.

Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean and the top is golden brown. The edges should be slightly pulling away from the sides of the pan.

Remove the cornbread from the oven and let it cool in the pan for at least 10-15 minutes before slicing and serving. This allows the cornbread to set properly.

Cut into 9 squares (approx. 7.5cm x 7.5cm or 3x3 inches) and serve warm. This cornbread is delicious on its own or with butter.