Preheat oven to 350 degrees F. Lightly spray a 9x13'' pan with cooking spray, then lay parchment paper on top, so it sticks firmly in the pan.

Place graham cracker sheets to cover the bottom of the pan.

Toffee layer: Add butter to a large saucepan over medium heat and allow it to melt. Add the brown sugar and pecans, and stir constantly, bringing the mixture to a full boil. Boil for 5 minutes, stirring constantly.

Bake: Remove from heat then pour mixture over graham crackers and spread into an even layer. Bake for 7 minutes.

Add chocolate: While the bars are baking, unwrap the Hershey's chocolate bars and break into pieces. Remove bars from oven and carefully place chocolate pieces across the top of the toffee layer. Cover the pan with a lid or aluminum foil to allow the chocolate to melt (5-10 minutes).

Smooth melted chocolate evenly over the bars.

Cool completely before serving. I like to store mine in the fridge to set up (not required) but I leave them out at room temperature for 15 minutes before cutting them, to allow the chocolate to soften.