Prepare the pan — Preheat the oven to 350°F. Line a 10 x 15-inch jelly roll pan with nonstick foil and lightly coat with cooking spray.
Arrange the crackers — Place the saltine crackers in a single, even layer across the pan, breaking crackers as needed so the pan is fully covered.
Make the toffee — In a 3-quart heavy-bottomed saucepan over medium heat, melt the butter and brown sugar together, stirring frequently.
Boil the toffee — Once the mixture comes to a boil, continue boiling for 3 full minutes, stirring almost constantly to prevent scorching.
Finish the toffee — Remove from heat and stir in the vanilla extract and salt.
Add the toffee layer — Immediately pour the hot toffee evenly over the crackers, spreading quickly with an offset spatula.
Bake — Bake for 5–7 minutes, until the toffee is bubbling vigorously.
Add the chocolate — Remove from the oven and sprinkle the chocolate chips evenly over the hot toffee. Let sit for 5 minutes, then gently spread into an even layer.
Add the toppings — Sprinkle holiday sprinkles or M&M’s over the melted chocolate while still warm.
Cool and set — Let the Christmas crack cool completely at room temperature, or refrigerate for at least 2 hours until fully set.
Break into pieces — Peel away the foil and break into pieces by hand or cut with a knife.