Preheat oven to 350 degrees F. Line a rimmed baking sheet with foil and mist the foil with cooking spray. Place the graham crackers all over the surface area of the pan, breaking them apart if necessary to fill in the gaps. Set aside.

In a medium saucepan, mix together the butter and brown sugar. Bring to a boil over medium heat and boil for 2 minutes, stirring constantly. Stir in the vanilla extract and chopped pecans and evenly pour the hot bubbly mixture over the graham cracker sheets. Working quickly, use a spoon or offset spatula to spread the mixture around to evenly coat the graham cracker sheets.

Bake for approximately 10 minutes or until the mixture is bubbly. Allow the bark to cool to room temperature before breaking into pieces. Store any leftovers in an airtight container.