Line a 9-inch x 13-inch baking sheet with foil. Arrange the graham crackers in a single layer over the foil. You may need to break the graham crackers to fill in the smaller spaces as best you can. Set aside.
Place a medium saucepan over medium heat, and melt the butter. Add the brown sugar. Cook until the sugar has dissolved and the mixture just begins to bubble.
Pour the sugar mixture over graham crackers. Use the back of a spoon or an offset spatula to spread the sugar evenly over the crackers. Immediately sprinkle the peanut butter chips all over the top.
Preheat the oven to 350 degrees F. Place the baking sheet into the oven while it preheats. Check on the candy every 3 to 5 minutes until the peanut butter chips look a little melted. Carefully remove the baking sheet.
Use an offset spatula or the back of a spoon to spread the melted peanut butter chips evenly over the graham crackers. Sprinkle the chopped pecans all over the top.
Let the toffee cool to the touch. Chill in the fridge for at least 1 hour or up to overnight. Break or cut the toffee into pieces before serving.
Notes
You can also use a 9x13 baking dish instead of a pan if needed. A pan with squared corners will work best, though.
Peanut butter chips don't melt the same way that chocolate chips do. They need the extra heat boost from the oven to melt enough to spread.