Preheat oven to 400°F (200°C). Line a 10x15 inch rimmed baking sheet with aluminum foil and spray generously with cooking spray.

Arrange saltine crackers in a single layer on the prepared baking sheet, fitting them together like a puzzle with no gaps. Break crackers as needed to fill all spaces completely.

In a medium saucepan over medium heat, melt butter and brown sugar together, stirring constantly until the mixture comes to a full rolling boil.

Once boiling hard, set a timer for exactly 3 minutes. Let the mixture boil for the full 3 minutes without stirring. The toffee will turn darker and smell like caramel.

After exactly 3 minutes, immediately remove from heat and pour the toffee evenly over the saltine crackers. Use a spatula to spread it quickly over all the crackers.

Place the baking sheet in the preheated oven and bake for 5-6 minutes, until the toffee is bubbling all over. Watch carefully and don't let it burn.

Remove from oven and immediately sprinkle chocolate chips evenly over the hot toffee. Let sit for 2-3 minutes to soften and melt.

Use an offset spatula or back of a spoon to spread the melted chocolate evenly over the entire surface. Add sprinkles, crushed candy canes, or chopped nuts now if using.

Let cool at room temperature for 30 minutes, then transfer to the refrigerator for at least 2 hours until completely firm and hardened.

Once fully hardened, lift the foil out of the pan and break the Christmas crack into irregular pieces. Store in an airtight container with parchment between layers.