Line a rimmed 10x15-inch baking sheet (jelly roll pan) with parchment paper or heavy-duty aluminum foil. Lightly spray with nonstick spray if needed. Preheat the oven to 350°F (175°C).

Lay the saltine crackers in a single, even layer on the prepared pan, salt 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, combine the butter and brown sugar. Cook, stirring frequently, until melted and starting to bubble. Once bubbling, cook for 3–4 minutes, stirring constantly, until the mixture thickens slightly and looks glossy.

Remove the saucepan from the heat and carefully stir in the vanilla extract and 1/4 teaspoon fine sea salt. The mixture may bubble up, so stir cautiously.

Immediately pour the hot toffee evenly over the saltine crackers. Working quickly, use an offset spatula or the back of a spoon to spread it so all the crackers are covered.

Place the pan in the preheated oven and bake for 8–10 minutes, or until the toffee is bubbling all over and slightly darkened around the edges. Do not let it burn.

Remove the pan from the oven and place it on a heatproof surface. Immediately sprinkle the chocolate chips evenly over the hot toffee. Let sit for 3–5 minutes until the chips look shiny and soft, then gently spread the melted chocolate into an even layer.

While the chocolate is still soft, sprinkle on your choice of toppings such as crushed candy canes, chopped toasted nuts, red and green sprinkles, mini M&Ms, and/or a light sprinkle of flaky sea salt.

Let the pan cool at room temperature for 20–30 minutes. Transfer to the refrigerator and chill for at least 1–2 hours, or until the chocolate and toffee are completely firm.

Once fully set, lift the parchment or foil out of the pan. Break the toffee into rustic pieces with your hands or cut into squares with a sharp knife. Serve, store, or package for gifts.