Preheat your oven to 350℉. Line your baking sheet with parchment paper.
Line the paper-covered baking sheet with graham crackers, keeping them close together so filling can't get into gaps in between them.
In your sauce pan, melt the butter over medium heat. Add in the brown sugar and stir constantly. You want this to boil (at around 230℉), but not burn. Maintain that heat for two minutes. This is almost, but not quite, making caramel.
Turn off the heat, add in the pecans and vanilla, and mix the filling well.
Spread the filling over your grahams as evenly as you can. Try not to crush the crackers or separate them as you spread it.
Bake this for ten minutes. The filling will spread out a bit and will be nice and bubbly.
Remove from the oven and let cool for two hours, until it is fully set. It should be easy to break apart along the graham crackers, for nice individual servings.