PREPARATION: Preheat oven to 400 degrees and set rack at center. Grease the cups of a muffin tin well, including around the top edges, or line with cupcake papers if you prefer. (I think muffins are better without papers as they bake with a crispier crust.)
MIX TOGETHER THE WET INGREDIENTS: In a medium bowl or large measuring cup, whisk together the cooled butter, eggs, sour cream, milk, and brown sugar. Set aside.
MIX TOGETHER THE DRY INGREDIENTS: In a large bowl, whisk together the flour, cornmeal, baking powder, baking soda, and salt.
STIR TOGETHER THE WET AND DRY INGREDIENTS; ADD PEACHES + JALAPEÑO: Pour the wet ingredients (sour cream mixture) into the dry ingredients (flour mixture), along with the peaches and jalapeño. Stir until just combined.
USING A SPRAYED LARGE SCOOP, divide the batter evenly into the greased muffin cups. Top each with some mozzarella cheese – about a tablespoon each.
BAKE FOR 14-16 MINUTES OR UNTIL GOLDEN BROWN and a toothpick inserted at center comes out clean or nearly clean. Set tin on cooling rack for 5 minutes. Bang pan firmly on the counter and flip to turn muffins out. Alternately, use a small off-set spatula or a small, thin knife to lift muffins out of the cups. Serve warm with butter. Cool, place in a large freezer bag and store a day or two at room temperature. To reheat: Slice in half, butter, and warm well in grill pan or skillet. Freeze for up to two months.