Butter two 11- x 15-inch shallow baking pans. Cover inside of pans with parchment paper; press paper into edges of pan. The butter helps to hold the parchment paper in place. Set aside.

Into a very large capacity bowl, pour the 18 cups of crispy rice cereal. Set aside.

In a 4 quart saucepot over medium heat, melt the butter. Add the corn syrup and brown sugar and stir well. Cook, stirring constantly, until mixture begins to boil. Boil and stir for three minutes.

Remove pot from heating element. Stir in the sweetened condensed milk. Return pot to cooking element; cook and stir until mixture comes to a boil once again. Boil and stir for an additional 3 minutes.

Carefully pour caramel sauce over the crispy rice cereal. Immediately stir the caramel sauce and cereal together until sauce is well distributed throughout the cereal.

Pour half of the mixture into each of the prepared pans. Spread evenly and pack mixture tightly together. A great way to compact the cereal mixture evenly is butter the outside bottom of a baking sheet, then set the buttered side on top of the cereal mixture. Press firmly all over to create a smooth, even surface.

If using cookie cutters, start cutting shapes while the cereal mixture is still warm. Space the cutouts closely together. Set each cut-out treat on wax paper, a silpat, or aluminum foil to cool.

If cutting squares, allow the mixture to cool in pans before cutting, about 2 hours.

Tip: If the cereal mixture becomes difficult to cut with cookie cutters, place the pan(s) in a warm (150-degrees F) for a few minutes to soften mixture.

Store treats in an airtight container. If storing stacked inside a container, place a sheet of wax paper between layers.