Preheat oven to 350°. If desired, combine peaches and bourbon; let stand 10 minutes., Meanwhile, place 1/4 cup butter in a 10-in. cast-iron or other ovenproof skillet; heat in oven until butter is melted, 5-7 minutes. Sprinkle brown sugar evenly over butter. Arrange peach slices over brown sugar., For batter, in a large bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Beat in egg and vanilla. In another bowl, whisk flour, baking powder and salt; add to creamed mixture alternately with milk, beating after each addition just until combined. Spread batter evenly over peaches., Bake until a toothpick inserted in center comes out clean, 40-45 minutes. Cool 5 minutes before inverting onto a serving plate. Serve warm.