How to brownies from a box/mix with a flaky top?
To get a shiny, flaky, crackly, light-colored crust on the top of your brownies, there are two effective changes you can make:

Beat the brownie batter for longer after adding the eggs.  Doing this increases the crust, which is actually meringue.  This is what you should try with your box mix, since the sugar is already included in  box.

Use white sugar (granulated sugar) with a from-scratch brownie recipe.  Brown sugar and corn syrup contain more moisture than white sugar, which will reduce the shiny crackly crust.