The secret to a crunchy brittle lies in the cooking process. If you don't cook the candy syrup until it reaches 305 degrees Fahrenheit, it won't harden no matter how long you cool it. A normal batch of peanut brittle poured into a buttered baking sheet will cool completely in about 30 minutes. At that point, you can hit the surface with a knife handle to crack the brittle, or simply lift up the pan a few inches and drop it onto the countertop. Place the cooling pan of brittle on a wire rack so air can circulate underneath it.

Thinner brittle cools more quickly. For faster cooling, pour the hot candy syrup mixture onto a couple of buttered baking sheets. Put on food-safe rubber gloves and, working quickly, stretch the still-pliable brittle into a thin layer. A pan of very thin brittle will cool in approximately 15 minutes.