Preheat the oven to 400°F. Combine the flour, cornmeal, baking powder, baking soda, and salt in a medium bowl. In a separate bowl, whisk together the buttermilk, eggs, oil, and brown sugar until smooth. Add the buttermilk mixture to the flour mixture and stir just until blended. Pour into a greased 10-inch ovenproof skillet and bake for about 20 minutes, until golden brown and solid in the center. Serve warm.