Preheat the oven to 400°F (200°C). Line a rimmed 10x15-inch baking sheet (jelly roll pan) with parchment paper or heavy-duty aluminum foil and lightly grease with nonstick spray or a thin layer of butter.

Lay the saltine crackers in a single layer on the prepared baking sheet, salt-side up, covering the entire surface. Break a few crackers if needed so they fit snugly with minimal gaps.

In a medium saucepan over medium heat, combine the unsalted butter and packed brown sugar. Cook, stirring gently and constantly, until the butter is melted and the mixture begins to bubble. Continue cooking at a gentle boil for 3–4 minutes, stirring constantly, until slightly thickened and glossy. Remove from heat and carefully stir in the vanilla extract and fine sea salt.

Working quickly, pour the hot toffee evenly over the layer of crackers. Use an offset spatula or the back of a spoon to spread it so all the crackers are coated.

Place the pan in the preheated oven and bake for 5–7 minutes, or until the toffee is bubbling all over and slightly darkened around the edges. Watch closely so it does not burn.

Remove the pan from the oven and let it sit for 1–2 minutes. Sprinkle the chocolate chips evenly over the hot toffee. Let stand for 3–5 minutes until the chips look shiny and soft, then gently spread the melted chocolate into an even layer. If needed, place the pan back into the turned-off warm oven for about 1 minute to help the chocolate melt.

While the chocolate is still soft, sprinkle your desired toppings evenly over the surface—such as chopped nuts, crushed candy canes, festive sprinkles, mini candies, and/or a pinch of flaky sea salt. Press gently so they adhere to the chocolate.

Allow the pan to cool at room temperature for 15–20 minutes. Transfer to the refrigerator and chill for 1–2 hours, or until the toffee and chocolate are completely firm. For a faster set, chill in the freezer for at least 30 minutes.

Once fully set, lift the slab out of the pan using the parchment or foil. Break into irregular bark-style pieces with your hands or a sharp knife. Serve immediately or store in an airtight container.