Preheat the oven to 350 degrees F. Spray sides of a 9 inch pie plate with nonstick cooking spray.
PREPARE THE PEACHES:
In a small bowl, combine the brown sugar and melted butter. Pour into the pie plate and arrange the peaches on top. Set aside.
PREPARE THE BATTER:
In a small bowl, combine the flour, baking powder, baking soda and salt. In a large bowl, use an electric mixer to beat the butter and sugar together until creamy. Add the egg and vanilla; beat until combined. Beat in the flour mixture and the buttermilk; beat until just combined.
Pour the batter into the pie plate and spread evenly over the peaches. Bake for 35 to 45 minutes, until a toothpick inserted into the center comes out clean. Cool for 10 minutes in the pan before inverting onto a serving plate.