Preheat the oven to 400°F (205°C). Line a 10 × 15-inch rimmed baking sheet with aluminum foil and lightly grease it. Add parchment over the foil if desired for easier removal.

Arrange the saltine crackers in a single layer across the pan, fitting them closely together. Break a few crackers if needed to fill gaps.

Add the butter and brown sugar to a medium saucepan over medium heat.

Stir until the butter melts and the mixture becomes smooth.

Bring the mixture to a full boil, then cook for 3 minutes, stirring constantly and scraping the bottom and sides of the saucepan.

Carefully pour the hot toffee evenly over the crackers and quickly spread it with a spatula.

Bake for 5 minutes, until the toffee is bubbling across the surface.

Remove the pan from the oven and let it stand for about 5 minutes.

Sprinkle the semi-sweet chocolate chips evenly over the hot toffee.

Let the chocolate chips sit for 2–3 minutes until softened, then spread the melted chocolate into an even layer.

Sprinkle the red and green M&M's over the chocolate, pressing them in lightly. Add holiday sprinkles if desired.

Let the candy cool until the chocolate begins to set, then refrigerate or leave at room temperature until completely firm.

Lift the candy from the pan, remove the foil or parchment, and break or cut it into about 20–24 pieces.