Preheat your oven to 350°F (177°C), and lightly spray a muffin pan with non-stick cooking spray.

Slice cold unsalted butter into 12 equal pieces and place one piece in each muffin cup. Scatter light brown sugar over the butter and arrange sliced peaches on top.

Chop remaining peaches into small cubes for the batter.

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

In a separate bowl, beat room-temperature butter and granulated sugar until light and fluffy. Add the egg and vanilla and mix until combined.

Gradually add the flour mixture alternating with buttermilk, mixing until just combined.

Fold in the chopped peaches.

Divide the batter evenly among the muffin cups, filling each about ¾ full.

Bake for 25-30 minutes or until golden brown and a toothpick comes out clean.

Cool in the pan for 5 minutes, then invert onto a wire rack.