Preheat your oven to 350°F. Line a large 15x10-inch jelly roll pan with parchment paper or a silicone baking mat. Arrange the Graham crackers in a single, tight layer, breaking them to fit so the entire bottom is covered.

While the oven preheats, spread the chopped pecans on a separate small tray and toast them for 5 minutes. Toasting the nuts before adding them to the syrup creates a much deeper, nuttier flavor.

In a medium saucepan, combine the butter and brown sugar. Bring to a boil over medium heat, stirring constantly. Once it reaches a rolling boil, let it bubble for exactly 3 minutes without stirring. This ensures the sugar reaches the "soft crack" stage for a perfect crunch.

Remove the pan from the heat and quickly stir in the vanilla extract (and cinnamon if using). The mixture will bubble up—this is normal.

Immediately pour the hot toffee mixture over the Graham crackers. Use an offset spatula to spread it quickly before it sets. Sprinkle the toasted pecans evenly over the top, pressing them down lightly with the spatula.

Bake for 10–12 minutes until the toffee is bubbling across the entire pan.

Remove from the oven. While still hot, melt the dark chocolate chips and drizzle them in a zigzag pattern over the pecans. Immediately sprinkle the flaky sea salt over the wet chocolate.

Let the bark cool completely at room temperature, then transfer to the refrigerator for 30 minutes to snap-set the chocolate. Break into rustic, irregular shards by hand.