Using Smith's egg nog, combine with peaches and let stand 10 minutes.
Meanwhile, place 1/4 cup butter in a 10-inch cast-iron or other ovenproof skillet; heat in oven until butter is melted, 5-7 minutes.
Sprinkle brown sugar evenly over butter. Arrange thawed peach slices over brown sugar.
For batter, in a large bowl, cream butter and sugar until light and fluffy. Beat in egg and vanilla.
In another bowl, whisk flour, baking powder and salt; add to creamed mixture alternately with Smith's 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.