Preheat oven to 350°F. Line a 9x13 baking pan with parchment, cutting slits in the corners so it fits flat in the pan.

Arrange graham crackers in a single layer over the parchment, breaking pieces as needed to cover the bottom completely. Broken pieces are fine; they will be hidden by the toffee and chocolate.

In a saucepan over medium heat, add butter and brown sugar. Cook, stirring occasionally, until the butter is melted, the sugar is mostly dissolved, and the mixture is smooth and starting to bubble. Remove from heat and stir in the vanilla.

Immediately pour the hot butter–sugar mixture evenly over the graham crackers and use a spatula to spread it to the edges so all the crackers are coated.

Bake at 350°F for 10–12 minutes, until the toffee layer is hot and bubbly all over.

Remove the pan from the oven. Sprinkle chocolate chips evenly over the hot toffee and let sit 3–5 minutes until the chips look shiny and soft. Gently spread the melted chocolate into an even layer.

While the chocolate is still soft, sprinkle chopped pecans over the top, if using. Press very lightly so they adhere.

Let the pan cool at room temperature about 30 minutes, then chill in the fridge 10–40 minutes, until the chocolate and toffee are set but not rock hard.

Lift the parchment out of the pan. Cut into bars with a sharp knife, or break into irregular “Christmas crack” pieces. Store in an airtight container.