Adjust oven rack to upper-middle position and preheat oven to 425 degrees F. Grease a 12-cup muffin tin (I used paper liners and the muffins stuck a bit, so I would recommend greasing the pan).
In a medium bowl, Whisk together 1½ cups of the cornmeal, the flour, baking powder, baking soda, and salt.
In a large bowl, whisk together the remaining ½ cup cornmeal and the milk. Microwave for 1½ minutes, then whisk thoroughly and continue to microwave, whisking every 30 seconds, until thickened to a batter-like consistency (the whisk will leave a distinct trail in the bottom of the bowl that slowly fills in), 1 to 3 additional minutes. [Note: If you do not have a microwave, you can do this step in a saucepan on the stovetop.]
Whisk in the sour cream, melted butter, and sugar until combined, then whisk in the eggs. Using a rubber spatula, fold in the flour mixture until thoroughly combined (the batter will be very thick). Divide the batter evenly among the prepared muffin cups (the batter will mound slightly above the rim).
Bake until the tops are golden brown and a toothpick inserted in the center comes out clean, 13 to 17 minutes, rotating the muffin tin halfway through baking. Let the muffins cool in the muffin tin on a wire rack for 5 minutes, then remove muffins from the muffin tin and let cool 5 minutes longer. Serve warm or at room temperature. Leftovers can be stored in an airtight container at room temperature for up to 3 days, or wrapped in plastic wrap, placed in a freezer bag and frozen for up to 2 months.
Cornmeal: Using coarse-ground or white cornmeal is not recommended.
Storing: Leftovers can be stored in an airtight container at room temperature for up to 3 days.
Freezing: Wrap cooled cornbread muffins in plastic wrap and place in a freezer-safe bag. Freeze for up to 2 months.
Thawing and Reheating: Remove the muffins from the freezer and allow to thaw at room temperature. Take out of the plastic wrap and warm in the microwave for about 30 seconds before serving. You can also re-heat the muffins in the oven at 350°F for about 10 minutes.