Step 1: Preheat your oven to 350°F (175°C). Line a large baking sheet (13x18 inches) with parchment paper, letting it overhang slightly on the sides. Arrange the saltine crackers in a single layer on the parchment-lined baking sheet, covering the entire surface as much as possible.
Step 2: In a medium saucepan, combine the unsalted butter and light brown sugar. Cook over medium heat, stirring frequently, until the butter is melted and the sugar is dissolved. Bring the mixture to a rolling boil and continue to boil for 3-5 minutes, stirring constantly to prevent burning, until the toffee mixture is a rich caramel color.
Step 3: Remove the saucepan from the heat and stir in the vanilla extract. Carefully pour the hot toffee mixture evenly over the saltine crackers, spreading it with a spatula to coat all crackers. Place the baking sheet in the preheated oven and bake for 5-7 minutes, until the toffee is bubbling and slightly darkened.
Step 4: Remove the baking sheet from the oven. Immediately sprinkle the chocolate chips evenly over the hot toffee. Let them sit for 5 minutes to soften and melt. Once softened, use an offset spatula or the back of a spoon to gently spread the melted chocolate evenly over the toffee layer.
Step 5: Sprinkle the flaky sea salt generously over the melted chocolate. If desired, add any additional toppings like chopped nuts or sprinkles at this stage. Transfer the baking sheet to the refrigerator and chill for at least 1 hour, or until the chocolate and toffee are completely set and firm.
Step 6: Once fully chilled, carefully lift the parchment paper from the baking sheet and break the Christmas Crack into irregular pieces. Store in an airtight container.