Preheat your grill to 425°Fahrenheit.
In a large bowl, whisk together cornmeal, flour, baking powder, salt, and sugar.
In another bowl, beat eggs, then add buttermilk and vegetable oil. Mix well.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not over mix.
Place the 1/4 cup butter in a 10-inch cast iron skillet and put it in the preheated grill for about 5 minutes, or until the butter is melted and the skillet is hot.
Carefully remove the hot skillet from the grill and swirl the melted butter to coat the bottom and sides.
Pour the excess melted butter from the skillet into the batter and stir quickly to incorporate.
Pour the batter into the hot skillet. It should sizzle as it hits the pan.
Place on the grill and cover. Cook for 25-30 minutes, or until golden brown on top and a toothpick inserted in the center comes out clean.
Let cool in the skillet for 5-10 minutes before slicing and serving.