Why does this always happen?

Why does this always happen when I bake brownies?

No, I don't mean why the brownie is almost gone... I know why that happens (yum).

Ever since I saw an episode of Oprah (favorite things) where she gave this brownie pan to her audience, I've wanted to bake brownies. But even though I use the best brownie recipe ever (Ben & Jerry's recipe) the top always separates from the actual brownie.. like this:

Even though I do enjoy picking of the top and snack on it, it would be nice to be able to cut pretty pieces, and not just crumbly pieces (especially if I were to share my brownie with friends and guests... I mean.... if that ever should happen... ah-hem).

I remember that I once read somewhere why it happens, and how to avoid it, but I simply can't remember. Can you help me?