Place an 8 inch cast iron skillet in your oven on the middle rack. Preheat the oven to 425℉.
To a large mixing bowl, add the tapioca, millet, sorghum, sugar, salt, baking powder, and baking soda. Whisk to combine.
150 g tapioca flour, 113 g millet flour, 113 g sorghum flour, 35 g cane sugar, 4 g salt, 2 tsp baking powder, 1 tsp baking soda
Add the cold butter to the flour mixture and work it in with your hands or a pastry cutter.
57 g cold butter
To a small mixing bowl, add the gf sourdough starter and egg and whisk to combine.
100 g gluten-free sourdough starter, 1 large egg
Add the starter/egg mixture to the flour/butter and stir to combine.
Gradually add the buttermilk until a dough forms and there is no dry flour left in the bowl.
227 g buttermilk
Add a little melted butter to the hot cast iron skillet and swirl it to coat the bottom. Dump the dough into the skillet and use an offset spatula to smooth the top.
1 tbsp melted butter
Using a knife or lame, cut a cross hatch on the top of the dough. Brush with melted butter and place in the oven. Bake 15 minutes, then turn the oven down to 400℉ and continue baking for 30 minutes.
Remove the bread from the skillet and place it on a wire rack to cool for at least 2 hours.