Preheat oven to 350°F. Line a large baking sheet with a Silpat baking mat, parchment paper, or aluminum foil. If using aluminum foil, spray it lightly with nonstick cooking spray.

Arrange all 15 graham cracker sheets in a single layer, touching, so they cover the baking sheet.

In a medium saucepan, combine 1 cup unsalted butter and 1 cup packed light brown sugar. Melt over medium-high heat, stirring until the sugar dissolves and the mixture comes to a boil.

Once boiling, maintain a steady simmer and cook for 4 minutes, stirring constantly.

Carefully pour the hot butter–sugar mixture evenly over the graham crackers. If needed, use a spatula to gently spread the mixture so the crackers are coated. Bake for 8 minutes.

Remove the pan from the oven and immediately sprinkle 1 1/2 cups chocolate chips evenly over the top.

Let the chocolate chips sit for about 2 minutes to soften and melt, then spread the melted chocolate evenly with a knife or rubber spatula.

Sprinkle 1/2 cup chopped almonds or pecans evenly over the melted chocolate.

Let the toffee cool until the chocolate hardens: about 2 hours at room temperature, or about 1 hour in the refrigerator, or about 30 minutes in the freezer.

When the chocolate is set, break the toffee into pieces and enjoy.