That's the great thing about this recipe - no candy thermometer is necessary. I give temperature guidelines in the recipe card below if you feel more comfortable using a candy thermometer but it's not required. If you are following the recipe using the timing guidelines you'll want to make sure that you look for the following clues: - when the butter and brown sugar have boiled long enough, the color will become caramel. - when baking the toffee, you want to make sure the topping is bubbling all over, including the center. The longer the mixture is cooked - whether on the stovetop or in the oven - the crispier the toffee will be. Shorter periods of time can cause the toffee to be chewy.