Preheat oven to 350°F (177°C). Melt butter in a cast iron pan over low heat. Add diced jalapeño and cook for 1 minute to soften and infuse flavor. Let cool slightly.
In a large bowl, whisk the cooled butter with buttermilk, eggs, and sugar until smooth.
In a separate bowl, mix cornmeal, flour, baking soda, and salt. Add to the wet ingredients and stir until just combined.
Pour the batter into the greased skillet or baking dish. Smooth the top.
Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean.
Let cool slightly before slicing and serving.