Preheat the grill for indirect cooking over medium heat (about 350°F | 180°C). I prefer to set my indirect zone with heat on either side of it for even cooking, but if this isn't available to you, rotate the skillet once during cooking so that it heats evenly.
Butter the cast iron skillet.
In a large bowl, whisk the flour, cornmeal, baking soda, and salt.
In a liquid measuring jug, beat the egg, then whisk in the buttermilk, melted butter, and sugar.
Pour the liquid ingredients into the dry ingredients and mix until everything is incorporated and no dry patches remain.
Spread the batter in an even layer in the skillet.
Cook on the grill until the cornbread is puffed and golden and a tester inserted in the thickest part comes out clean, 22 to 25 minutes. Serve warm.
Notes
If you prefer a sweeter cornbread, increase the sugar to 2 ½ tablespoons
Leftover cornbread makes great croutons! Cut your cornbread into cubes and toast in a 350°F oven until golden, about 15 minutes, or cook gently in your skillet over low heat, stirring occasionally, until golden all over. Add them to salads or plop them on top of your favorite Dutch oven chili.
Avoid opening the grill lid frequently as this will cause the temperature inside the grill to fluctuate.