I buy and make box brownies all the time, but the top crust always ends up looking more like the topping of a cake, rather than the flaky top shown in the picture on the box. It's pretty easy to follow the 4 steps on the box; is there something I am missing?
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.
Also, try using butter instead of oil. Using butter, as in flaky pastries, will help promote pockets of fat and stream that will create those flaky and moist brownies you're looking for.
You might want to add more leavening power. This will help the brownies rise more and will help separate the layers of brownie.