Preheat the oven to 400°F (204°C). Line a rimmed half-sheet pan (13 x 18 inches) with foil, then a sheet of parchment on top. Arrange the saltines in a tight single layer.

In a medium saucepan, combine the butter and brown sugar over medium heat. Stir as the butter melts, then bring to a steady boil and cook for 3 minutes, stirring often, until thick, glossy, and bubbling all over.

Remove from heat and stir in the vanilla and kosher salt. Immediately pour the hot toffee over the crackers and spread quickly to the edges with an offset spatula.

Bake for 5 minutes, or until the toffee is vigorously bubbling across the entire surface and looks slightly caramelized at the edges.

Remove the pan to a cooling rack. Scatter the chocolate chips evenly over the hot toffee and let sit for 1 to 2 minutes until softened, then spread into a smooth layer.

Sprinkle with chopped pecans, Christmas sprinkles, and a pinch of flaky sea salt, if using. Gently press toppings to help them adhere.

Cool at room temperature for 30 to 45 minutes, then refrigerate for 20 to 30 minutes until the chocolate is fully set and the slab feels firm.

Lift the parchment to remove the slab from the pan. Break into rustic pieces or cut with a sharp knife. Serve or store in an airtight container.