Preheat oven to 350ºF. Grease and flour a 12-serving square muffin pan, also known as a brownie pan.* (You can also use a traditional round muffin pan. The baking time may be a little shorter.)
In a large bowl, whisk together flour, sugar, cornmeal, baking powder and salt.
In a separate bowl, whisk eggs, sour cream, oil and butter until smooth.
Add egg mixture into the flour mixture and mix just until combined. A few lumps are okay.
Transfer batter to prepared pan, filling wells halfway full. (Fill 2/3 full if using a round muffin pan.)
Bake until edges turn golden brown and a toothpick inserted into center comes out clean, about 17-20 minutes. Do not over bake.
Let cool in pan for 5 minutes them transfer muffins to a cooling rack. Serve warm or room temperature.