Step 1

Preheat the oven to 350°F. Line a half sheet pan with parchment paper or aluminum foil. If using aluminum foil, spray lightly with nonstick cooking spray.

Step 2

Place the graham crackers tightly in a single layer, breaking pieces as needed so that the entire pan is covered. Set aside.

Step 3

Place the butter and brown sugar in a medium saucepan, and cook over medium-high heat until the mixture comes to a boil. Continue cooking, stirring constantly and gently until the mixture has thickened and reached a temperature between 270 and 290°F. (This should take 8 to 10 minutes.)

Step 4

Immediately pour the butter mixture over the graham crackers. Working quickly, spread evenly.

Step 5

Bake for 6 to 8 minutes. The topping should be bubbling slightly.

Step 6

Remove the pan from the oven. Immediately sprinkle the chocolate chips evenly over the graham crackers. Let them sit for 2 to 3 minutes until softened enough to spread, and then smooth evenly. Sprinkle the nuts on top.

Step 7

Leave the toffee bars at room temperature until set (about 2 hours). Alternatively, you can refrigerate them for 1 hour or freeze 30 minutes.

Step 8

Break into pieces once set.