Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper and arrange graham crackers in a single layer to cover the bottom.
In a medium saucepan over medium heat, melt butter and brown sugar together. Stir until melted and combined, then bring to a boil. Boil for 3 minutes, stirring constantly.
Pour the hot brown sugar mixture over the graham crackers and spread evenly. Bake for 5 minutes, until bubbly.
Immediately sprinkle chocolate chips over the hot toffee and let sit for a minute to melt. Spread the melted chocolate evenly over the top.
Sprinkle chopped pecans over the melted chocolate. Let cool in the pan for about 20 minutes, then refrigerate until completely cooled and chocolate is set. Break into pieces and serve.
Notes
You can store these toffee bars in an airtight container for up to a week or freeze for longer storage.