Preheat the oven to 350°F. Grease a 12-cup muffin tin with 1 tablespoon of butter.
In a small bowl, toss the sliced peaches with lemon juice and set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon (if using).
In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Gradually add the dry ingredients to the butter mixture, alternating with the buttermilk, starting and ending with the dry ingredients. Mix until just combined.
Spoon a small amount of batter into each muffin cup, just enough to cover the bottom. Place a few peach slices on top of the batter in each cup, then fill the cups with the remaining batter until they are about 3/4 full.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
Allow the mini cakes to cool in the pan for 5 minutes before inverting them onto a wire rack to cool completely.