Whisk together the cake flour, all-purpose flour, sugar, baking soda and salt into a large bowl. Make a well in the middle. Combine the melted butter with the buttermilk and whisk until blended. Add the buttermilk mixture to the flour. Stir with a fork just until the dough begins to come together.
Using your hands, gently knead the dough (still in the bowl) until it starts to come together; it should still be a little bumpy. Do not over-knead the dough as it will turn out tough. Once all the flour is incorporated, pat the dough into a round about 6-inches in diameter and about 2-inches high. Place the loaf in a lightly greased cast iron skillet or on a parchment lined baking sheet. Cut a cross shape into the top of the dough.
Bake until golden brown or when a skewer inserted into the center of the bread comes out clean, about 40 to 45 minutes. It should reach 170°F in the center using an instant read thermometer. Tent the loaf with foil during baking if it is browning too much.
Brush with melted butter if desired and cool about 30 minutes before serving.
Notes
We use full-fat buttermilk in this recipe. This buttermilk is very thick so you may need a little more liquid to get the dough to come together. You want it to be soft, but not wet. A little sticky is fine. If using low-fat buttermilk, you may need a few extra tablespoons of flour for it to come together. Use your best judgment and adapt accordingly.
The deeper you make the cut in the top, the wider the bread will spread
Store leftovers at room temperature in a sealed container.
Nutrition information is calculated using an ingredient database and should be considered an estimate. When multiple ingredient alternatives are given, the first listed is calculated for nutrition. Garnishes and optional ingredients are not included.