Bake cake in 9x9 or 8x8 inch pan according to package directions. Allow to cool.
Combine cherries with sugar and balsamic. Allow to soak and macerate for at least 2 hours.
Combine half of the liquid from the cherries with the mascarpone and blend well. Spread on top of cake.
Using slotted spoon, add cherries to top of cake. Drizzle remaining liquid over the cherries.
Serve immediately or store in an airtight container in the refrigerator.
Notes
I used Trader Joe's mix which makes an 8x8 inch cake. Other cake mixes might result in a larger cake and require different ingredients other than the egg, butter and milk.