Preheat oven to 350.
Grease 12 muffin turns with cooking spray.
Cut the reserved 3 Tbsp of butter into pieces and place in bottom of each muffin tin evenly.
Sprinkle the brown sugar evenly amongst the 12 muffin tins.
Place 3 slices of peaches in each tin. Chop the remaining peach slices into pieces for the batter.
In an electric mixer, cream butter and sugar.
Mix the remaining dry ingredients together.
Fold in remaining peaches.
Top each muffin tin with batter- do not over fill.
Bake for 20 minutes.
Cool completely before turning out onto a dish.