The cracks indicate when to take the brownies out of the oven.
The brownies are ready when the surface starts to crack and the center holds firm when the pan is shaken.
Whether using a pre-made mix or a family recipe, baking time determines the final texture. Overbaked brownies become burnt and rock hard, instead of moist and creamy. Oven temperatures can be unpredictable, and instructions vary, so it's best to rely on a visual indicator: the surface. This rule outweighs the specific recipe instructions, so it's wise to check your brownies well before the timer goes off.
The definition of a perfect brownie varies from person to person. The longer it's baked, the more cake-like and less creamy it becomes. That's why the toothpick test doesn't work with creamy brownies: if the toothpick comes out clean, they're probably overbaked. Nor does gently tapping the surface to see if it springs back is effective, because creamy brownies don't spring back. Instead, when you tap a light, slightly crispy layer that's starting to crack, it should deflate a little, crack further, and hold its shape.
The edges darken and pull away slightly from the pan. It's best to remove the brownies before the center becomes too firm and the edges brown too much, as they continue to cook outside the oven.
The rack also makes a difference. On the bottom rack, brownies can get too crispy too quickly; a tray on the middle rack gives balanced, perfectly creamy results.
