The edges of brownies cook faster than the center due to the fact that they bake from the outside in. The result is a border that is crisp and chewy, while the middle is softer and fudgier. When brownies are allowed to cool completely before cutting, the edges will become firm and slightly crispy. Likewise, the temperature of your oven plays a role in crispiness; a hotter oven promotes a crispier edge, but don't overbake brownies as they will dry out and crumble.