Microwave butter in an 8-inch round glass baking dish at HIGH 30 seconds.
Spread softened butter to cover the bottom of the baking dish.
Sprinkle brown sugar over softened butter.
Arrange peaches and cherries on brown sugar.
Prepare cake mix according to package directions.
Remove 1/4 cup of cake batter.
Reserve for cupcakes.
Spread remaining cake batter over peaches and cherries.
Microwave at HIGH 9 to 12 minutes.
Rotate dish 1/2 turn after 4 minutes.
Let cake stand 5 to 10 minutes.
Invert cake onto serving platter.