Preheat oven to 325°F (163°C). Line a large rimmed baking sheet with foil or parchment and lightly spray with non-stick spray.
Arrange the saltine crackers in a single even layer on the pan.
In a saucepan over medium-low heat, melt the butter and brown sugar together. Stir and bring to a gentle boil, cooking for 3 minutes.
Pour the hot caramel mixture evenly over the crackers. Spread quickly with a spatula.
Bake for 8 minutes, until the caramel is bubbling.
Remove from oven and immediately sprinkle chocolate chips over the top. Wait 1–2 minutes for them to soften, then smooth with a spatula.
Add M&Ms, nuts, or other toppings.
Freeze for 15 minutes, then rest at room temperature for 15 minutes.
Flip the slab onto a cutting board, peel off the foil, and break into pieces.