Prepare a 12 cup muffin pan by spraying with non-stick cooking spray. Cut 1 tablespoon of the unsalted butter into 12 small cubes and place one in each cup. Evenly sprinkle the brown sugar into the bottom of each muffin cup.
Slice 2 of the peaches in half and remove the pit. Peel the peaches and slice into very thin slices. Place several of the peach slices in the bottom of each muffin cup.
In a medium mixing bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, with an electric mixer beat together the 1/3 cup unsalted butter with the sugar until light and fluffy. Add the egg and vanilla extract and beat until smooth. Add half of the flour mixture and beat until combined. Add the second half of the flour mixture and mix well. Stir in the buttermilk.
Remove the pit from the remaining two peaches. Peel the peaches and slice into small cubes. Fold into the cake batter.
Scoop the batter evenly into the muffin cups.
Bake at 350F for 25 minutes until a toothpick inserted into the center comes out clean. Cool the cakes on a wire cooling rack for 5 minutes, then serve with whipped cream.
Notes
Store cooled cakes in an airtight container in the fridge for up to 3 days. Warm them up for a few seconds in the microwave and add the toppings of your choice.