Place a 10" cast iron skillet on middle rack in oven and preheat to 425°F.

In a medium bowl, whisk together cornmeal, flour, sugar, baking powder, salt, and baking soda.

In a separate medium bowl, whisk together sour cream, buttermilk, eggs, 3 tablespoons melted butter, and canola oil.

Pour wet ingredients into bowl with dry ingredients and whisk until completely combined.

Using pot holders, carefully remove hot pan from oven.

Place 1 tablespoon butter in skillet and swirl to completely melt and coat inside of pan.

Pour in cornbread batter and place in oven. Bake until skewer inserted into middle of cornbread comes out clean, about 20 minutes.

Let bread cool in pan for 5 minutes, then carefully turn out to wire rack and let cool an additional 10 minutes.

Serve immediately. Reheat any leftover cornbread before serving again.