Line a 13 x 18 inch baking sheet with parchment paper. Arrange the graham crackers in an even single layer, breaking pieces as needed to fit.
In a medium saucepan, melt the butter over medium heat. Add the brown sugar and salt, then stir constantly until the mixture begins to bubble. Let it cook for 2 to 3 minutes until slightly thickened. Remove from heat and add the vanilla.
Carefully pour the hot butter brown sugar mixture over the graham crackers. Spread it gently with a spatula so every cracker is coated.
Sprinkle the chopped pecans evenly over the top.
Bake at 350 degrees F for 8 to 10 minutes or until the toffee is bubbling across the entire surface.
Let it cool completely, then break into pieces or cut evenly with a sharp knife (the method I prefer). Store in an airtight container.