Whisk together flour, cornmeal, sugar, baking powder, salt, and baking soda. Store in a tightly sealed container in a cool, dry place for up to 3 months.
When ready to bake, preheat oven to 400 F. Transfer mixture to a large bowl and whisk in milk, oil, and eggs until evenly combined. Pour batter into a lightly greased baking vessel and bake as directed below. Let cool slightly before serving.
Cooking Times: Mini muffin tin (makes 45): 10 to 12 minutesStandard muffin tin (makes 15): 12 to 15 minutes10-inch cast-iron skillet: 25 to 30 minutes8-by-8-inch baking dish: 30 to 35 minutes8-by-4-inch loaf pan: 55 to 60 minutes (tent with foil if browning too quickly)