Step 1: Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet (15x10 inch) with parchment paper or a silicone baking mat.
Step 2: Arrange the saltine crackers in a single layer on the prepared baking sheet, fitting them as closely together as possible.
Step 3: In a medium saucepan, combine the butter and brown sugar. Cook over medium-high heat, stirring constantly, until the mixture comes to a boil.
Step 4: Once boiling, continue to boil for exactly 3 minutes without stopping your stirring. This ensures the toffee sets correctly.
Step 5: Immediately pour the hot toffee mixture over the crackers and spread it evenly using a spatula to cover all the crackers.
Step 6: Place the baking sheet in the oven and bake for 5 minutes, or until the toffee is bubbling all over.
Step 7: Remove the pan from the oven and immediately sprinkle the chocolate chips evenly over the hot toffee.
Step 8: Let the chocolate sit for 2 minutes to soften, then use a spatula to spread the chocolate into an even layer.
Step 9: If using nuts or sprinkles, add them now while the chocolate is still wet. Let the tray cool to room temperature, then refrigerate for at least 2 hours until completely set.
Step 10: Once hardened, break the christmas crack into irregular pieces and store in an airtight container.