Heat oven to 350°F. Line a rimmed baking sheet with parchment or foil, reaching up the sides for easy lifting later.

Arrange the saltines or grahams in a snug single layer to cover the sheet. Fill small gaps with broken pieces.

In a saucepan over medium heat, melt the butter with the brown sugar, stirring often. When the mixture reaches a steady bubble, cook about 3 minutes until slightly thick and glossy. Remove from heat.

Pour the hot toffee over the crackers and spread quickly to the edges. Bake 15 minutes, keeping an eye on the color. If the edges darken early, reduce to 325°F for the last minutes.

Take the pan out. Sprinkle chocolate chips over the hot toffee and let them sit 5 minutes. Spread into an even layer with an offset spatula or the back of a spoon.

While the chocolate is still soft, add pretzels, coconut, and M&Ms. Press lightly so they set into the chocolate. Drizzle white chocolate or add chopped nuts if using. A small pinch of flaky sea salt makes a balanced finish.

Cool at room temperature about 2 hours, or refrigerate 30 minutes until firm. Lift the slab out by the parchment and break into pieces.