Preheat oven to 400°F (204°C). Generously spray a half-sheet pan with cooking spray or line with a silicone baking mat or parchment paper. Arrange crackers on top as pictured. Set aside.

Stirring lightly, bring butter and brown sugar to a boil in a large saucepan over medium heat. As it begins to boil, leave it alone without stirring for 3 minutes. Pour boiling mixture evenly over crackers, then bake for 5 minutes.

As toffee bakes, melt peanut butter in microwave. Remove toffee from the oven, then pour peanut butter over top. Smooth it into an even layer. Immediately sprinkle chocolate chips on top, then return the pan to the oven for 1 minute to help chocolate chips melt. Remove from the oven and spread chocolate chips into one melted chocolate layer. Top with M&Ms.

Refrigerate toffee for 2 hours. Once set, break into pieces.