Set oven to 350°F (175°C). Cover a rimmed baking sheet with aluminum foil and apply a light coating of butter.
Place graham crackers in one even layer over the baking sheet, breaking or cutting pieces as needed to form a continuous base that reaches all edges.
In a medium saucepan, mix together 1 cup (226 g) unsalted butter, 1 cup (220 g) brown sugar, the chopped pecans, and ¼ teaspoon salt.
Place over medium heat and stir without stopping until the mixture comes to a full rolling boil. Maintain a boil for exactly 2 minutes, stirring constantly to avoid scorching and to achieve uniform caramelization.
Take off the heat and immediately blend in 1 teaspoon (5 ml) vanilla extract until thoroughly incorporated.
Spread the hot caramel over the graham cracker base, using a silicone spatula to distribute it evenly and cover the entire surface.
Place in the preheated oven and bake for 10–12 minutes, until the edges turn golden brown and small bubbles form.
Take out of the oven and allow to cool entirely at room temperature until the caramel hardens and becomes crisp.
After it has cooled and hardened completely, snap the bark into uneven, bite-sized shards.