Heat the oven to 350°F, then cover a rimmed sheet pan with aluminum foil. Use a pan with raised edges so the hot topping stays contained.

Butter the foil lightly and arrange the graham crackers across the pan, breaking pieces to fill open spaces. The crackers should form one even layer with no overlapping corners.

Place the butter, brown sugar, chopped pecans, and salt in a medium saucepan over medium heat and stir continuously until the mixture reaches a rolling boil. Continue stirring for 2 minutes as the bubbles become vigorous and the color turns a deeper amber brown.

Take the saucepan off the heat and mix in the vanilla, then pour the hot pecan toffee over the crackers. Spread it promptly to the pan edges with a spatula until the crackers are covered and the pecans are evenly scattered.

Bake for about 10 minutes. The surface should bubble all over the pan and give off a buttery, toasted pecan aroma.

Let the bark cool completely before breaking it into pieces. Wait until the bottom of the pan feels cool to the touch so the candy hardens into clean, crisp shards.