Step 1: Preheat your oven to 350°F (175°C). Line a large baking sheet (13x18 inches) with parchment paper, extending up the sides. Arrange the graham cracker sheets in a single layer across the parchment-lined baking sheet, breaking them as needed to fit snugly and cover the entire bottom.
Step 2: In a medium saucepan, combine the butter and brown sugar. Cook over medium heat, stirring constantly, until the butter is completely melted and the sugar is dissolved. Bring the mixture to a rolling boil and continue to boil, stirring constantly, for 3-5 minutes. The mixture should thicken slightly and turn a rich caramel color.
Step 3: Remove the saucepan from the heat and stir in the vanilla extract. Immediately and carefully pour the hot toffee mixture evenly over the graham crackers, spreading quickly with a spatula to cover all the crackers.
Step 4: Transfer the baking sheet to the preheated oven and bake for 5-7 minutes, or until the toffee is bubbly and slightly darker in color.
Step 5: Remove the baking sheet from the oven and immediately sprinkle the chocolate chips evenly over the hot toffee. Let sit for 5 minutes to allow the chocolate to melt. Then, use an offset spatula or the back of a spoon to spread the melted chocolate evenly over the toffee.
Step 6: If using, sprinkle the chopped pecans (or other desired toppings) over the melted chocolate. Gently press them down slightly.
Step 7: Allow the toffee to cool completely at room temperature for at least 1-2 hours, or transfer to the refrigerator for 30-60 minutes to speed up the cooling and hardening process. Once completely firm, break the toffee into irregular pieces. Store in an airtight container.