Preheat cast iron pan - Preheat the oven to 400°F with the skillet inside.
Melt the butter - Melt 6 T butter in the microwave and set aside to cool.
Make the batter - Whisk together all of the dry ingredients in a large bowl (1 1/2 cups cornmeal, 1/2 AP flour, 1 t baking soda, 1 t salt, 1 T sugar). In another bowl, beat the eggs and 1 1/4 cup of milk until combined, then mix that into the bowl of dry ingredients. Stir in the melted butter, then fold in the optional add-ins.
Pour batter into the hot skillet and bake - When the oven is hot, pull out the pre-heated skillet and add 1 T of butter. Swirl the butter to coat the pan, then pour in the the cornbread batter, smoothing the top to ensure that it's evenly distributed. Bake for about 20 minutes, or until the edges are beginning to brown and a toothpick inserted in the center of the bread comes out clean.
Rest bread in skillet, then serve - Let the bread rest for at least 10 minutes in the skillet before cutting it into wedges and serving.