Preheat your oven to 400°F (200°C). Line a rimmed baking sheet with aluminum foil, then place a sheet of parchment paper on top. The foil provides structure, and the parchment prevents sticking.
Place the saltine crackers in a single, tight layer on the prepared pan, salted side up. Cover as much of the surface as possible, breaking a few crackers if needed to fill small gaps.
In a medium saucepan over medium heat, melt the butter. Add the brown sugar and stir gently until the mixture is combined and smooth. Once it begins to bubble, continue stirring and bring it to a low, steady boil.
Let the mixture bubble for 3–4 minutes, stirring constantly, until it looks thick and glossy and pulls slightly away from the sides of the pan. If using a candy thermometer, it should reach about 270–290°F.
Remove the saucepan from the heat and carefully stir in the vanilla extract (it may hiss). Immediately pour the hot toffee mixture evenly over the saltines, using a spatula to spread and cover every cracker. Work quickly, as it begins to set fast.
Place the pan in the preheated oven and bake for about 5 minutes, or until the toffee is bubbling all over. Watch closely to prevent scorching, as ovens can vary.
Remove the pan from the oven and immediately sprinkle the chocolate chips evenly over the hot toffee layer. Let sit for 3–5 minutes to soften, then use an offset spatula or the back of a spoon to spread the melted chocolate into a smooth, even layer.
While the chocolate is still soft, sprinkle on your chosen toppings such as crushed candy canes, chopped nuts, festive sprinkles, mini candies, and/or flaky sea salt. Gently press larger pieces so they adhere to the chocolate.
Let the pan cool at room temperature for 20–30 minutes, then transfer it to the refrigerator to chill for at least 1–2 hours, or until completely firm.
Once fully set, lift the entire sheet of toffee out of the pan using the foil. Break into irregular pieces with your hands or cut into squares with a sharp knife. Serve on platters, add to cookie trays, or pack into tins or bags for gifting.