Line a large baking sheet with aluminum foil. Lightly grease with nonstick cooking spray. Cover baking sheet with whole graham cracker sheets.
In a medium saucepan set to medium heat, bring brown sugar and butter to a boil. Simmer for 4 minutes, stirring constantly. Pour over graham crackers and bake for 8 minutes.
Remove from oven and immediately sprinkle chocolate chips over top. Let sit for 2 minutes. Spread melted chocolate with a knife or rubber spatula. Sprinkle pecans over top.
Refrigerate for at least 1 hour, or freeze for 30 minutes, then cut and serve toffee.