Heat the oven to 425° F. Line a 12-cup muffin tin with paper liners, or grease the cups well.
In a large bowl, whisk together the cornmeal, flour, sugar, baking soda, cream of tartar, and salt.
In a medium bowl, whisk together the sour cream, milk, eggs, and bacon fat or butter.
Add the liquid ingredients to the dry and stir together quickly, stopping as soon as everything is combined. Working quickly, divide the batter evenly among the muffin cups. Bake for 12 to 15 minutes, until a toothpick inserted into the middle of a muffin comes out clean. Cool in the tin for 5 minutes, then turn the muffins out onto a rack to finish cooling. Serve warm with salted butter. (The muffins will keep for a few days in an airtight container. I recommend splitting, toasting, and buttering them once they're more than a day old.)