S'Mores Granola Bars

4 cups rolled oats (regular- not quick cooking oats)
4 cups Rice Krispies cereal
¾ cup granulated sugar
½ cup (1 stick) unsalted butter
¼ cup water
½ cup corn syrup
1 tsp salt
3 cups miniature marshmallows, divided
1½ cups chocolate chips
1. Preheat oven to 425°F. Lightly grease a half-sheet (18×13-inch) pan, or two 9×13-inch pans.
2. Place the oats on the prepared pan(s). Bake for 3 minutes- stir oats- bake for 3 more minutes. The oats won’t brown, but they should begin to smell toasty. Don’t overbake, or the oats will begin to burn. Remove the pan from the oven; transfer the oats to a bowl and add the Krispies. (Keep the oven on).
3. In a large saucepan, combine sugar, butter, water, corn syrup, and salt. Bring the mixture to a boil and boil for 5 minutes, or until the temperature reaches 250°F. on a candy thermometer. Remove the mixture from the heat and pour it over the oats and cereal, tossing to combine. Add 1½ cups of the marshmallows, and stir again until well combined.
4. Place the mixture on the prepared pan (the one you used for the oats), patting it flat. Sprinkle with the remaining 1½ cups of marshmallows and chocolate chips. If you grease the bottom of another half-sheet pan, you can use it to press down on the mixture to make it flat and even.
5. Return the pan of bars to the oven for 3 minutes, or until the marshmallows have puffed and are slightly browned. Be careful… the marshmallows will burn if you leave them in too long. Remove the bars from the oven and cool for about 10 minutes on a rack. Cut into squares while still warm- it’s easiest to do this using a baker’s bench knife or a knife with a wide blade.