In a medium bowl, combine the flour, baking powder and salt; set aside. In a large bowl, beat 1/2 cup butter with an electric mixer on medium to high speed for 30 seconds. Beat in granulated sugar until well combined. Add egg and vanilla, beating until combined. Alternately add the flour mixture and milk to beaten butter mixture, beating on low speed after each addition just until combined. Spread batter evenly over the peaches in the pan.