Preheat oven to 375°. Put butter in a 10-in. cast-iron frying pan and set in oven. (It's okay if the butter browns a bit; it will only add flavor.)
Whisk dry ingredients in a large bowl and mix eggs, sugar, and buttermilk in another bowl.
Remove pan from oven and brush butter over the inside.
Whisk wet into dry ingredients, pour in melted butter from pan, and whisk until batter is smooth.
Pour batter into hot pan and bake until lightly browned and springy to the touch, 25 to 30 minutes.
*Find red cornmeal on starkeroundbarn.com; blue and yellow cornmeals are widely available.