Preheat the oven to 350°F (175°C). Line a half sheet pan with parchment paper or aluminum foil. If using aluminum foil, spray lightly with nonstick cooking spray.
Place the graham crackers tightly in a single layer, breaking pieces as needed so that the entire pan is covered. Set aside.
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 (132-143°C). (This should take 8 to 10 minutes.)
Immediately pour the butter mixture over the graham crackers. Working quickly, spread evenly.
Bake for 6 to 8 minutes. The topping should be bubbling slightly.
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.
Leave the toffee bars at room temperature until set (about 2 hours). Alternatively, you can refrigerate them for 1 hour or freeze 30 minutes.
Break into pieces once set.
Notes
The yield will vary depending on how large or small your pieces are.
Store in an airtight container at room temperature up to 2 weeks.