Preheat oven to 425 with the cast iron pan in the oven.
In a large mixing bowl, whisk together flour, cornmeal, baking powder, and salt.
In a medium bowl, melt 3 tbsp of butter. The whisk in the buttermilk and egg.
Pour buttermilk mixture into the flour mixture and whisk until just combined.
Fold in cheddar and jalapeños with a rubber spatula.
Add remaining 3 tbsp of butter to the hot cast iron pan and put back in the oven until butter is melted and bubbly.
Remove pan from the oven and pour in cornbread mixture.
Place back in oven and bake for 20-25 minutes.
