Line a 10x15-inch rimmed baking sheet (jelly roll pan) with parchment paper or heavy-duty foil, then lightly spray with nonstick spray. Preheat the oven to 400°F (200°C).

Arrange the saltine crackers in a single, tight layer over the lined pan, covering the whole surface edge to edge. Break a few crackers as needed to fill small gaps, avoiding large overlaps so the toffee bakes evenly.

In a medium saucepan over medium heat, combine the butter and brown sugar. Cook, stirring often, until the butter melts and the mixture comes together. Bring to a gentle boil and let it bubble for about 3 minutes, stirring occasionally, until slightly thickened and glossy.

Watch for steady, even bubbles across the surface and a syrupy texture. Remove from heat and carefully stir in the vanilla and fine sea salt (if using); the mixture may hiss slightly.

Working quickly, pour the hot toffee mixture evenly over the saltines. Use an offset spatula or the back of a spoon to spread it and nudge it into the corners; it doesn’t need to be perfect, as it will spread more in the oven.

Place the pan in the preheated oven and bake for 5–7 minutes, until the toffee is bubbling up over and between the crackers and is deep golden. Watch closely near the end so it doesn’t burn.

Remove the pan from the oven and place it on a heat-safe surface. Immediately sprinkle the chocolate chips evenly over the hot toffee layer. Let sit for 3–5 minutes, until the chips look glossy and soft.

Using a clean offset spatula or the back of a spoon, gently spread the melted chocolate into an even layer over the toffee. Swirl lightly if desired for a decorative look.

While the chocolate is still wet, sprinkle on chopped nuts, holiday sprinkles, crushed candy canes, mini chips, and/or flaky sea salt as desired. Press very gently to help the toppings adhere.

Let the pan cool at room temperature for 20–30 minutes, then transfer to the refrigerator for 1–2 hours, or until completely firm. For faster setting, you can chill it in the freezer, checking every 20–30 minutes.

Once fully set, lift the toffee out of the pan using the parchment or foil. Break into rustic pieces with your hands or a sharp knife. Serve on a holiday platter or pack into tins for gifting.