Preheat the oven to 400°F/205°C. Grease a 9-inch cast iron skillet or 9-inch baking dish with butter and place the skillet/dish into the oven to heat up.
In a large bowl, whisk together the flour, cornmeal, sugar, baking powder, baking soda and salt. Set aside.
In a medium bowl, whisk the melted butter, sugar together until combined, then mix in eggs and buttermilk until smooth. Pour the wet ingredients into the dry ingredients and whisk until combined. Don’t overmix.
Optional: At this point, mix in your add-ins (corn kernels, cheddar cheese, nuts, jalapeno peppers, etc) and gently fold into the batter.
Carefully remove your hot pan or dish from the oven with oven mitts, and pour the batter into the hot skillet or dish. Bake for about 20-25 minutes, until the cornbread begins to brown on top and a toothpick inserted in the center comes out clean. Allow to cool before slicing and serving.