Melt butter in a 10 inch ovenproof skillet or a 9x9 inch square pan. Sprinkle brown sugar over melted butter. Arrange peach slices on brown sugar.
Beat remaining ingredients with electric mixer on low speed for 30 seconds, scraping bowl. Beat on high for 3 minutes, scraping bowl occasionally. Pour over peaches.
Bake for 45-50 minutes in skillet or 50-55 minutes in square pan. (or until toothpick inserted in middle comes out clean.)
Immediately turn cake upside down onto a heatproof plate. Let skillet or pan remain over cake a few minutes so brown sugar topping can drizzle over cake. Cool for 15 minutes before serving.