4 cups (14 ounces) rolled oats
4 cups (4 1/2 ounces) Rice Krispies cereal
3/4 cup (5 1/4 ounces) sugar
1/2 cup (1 stick, 4 ounces) unsalted butter
1/4 cup (2 ounces) water
1/2 cup (5 1/2 ounces) corn syrup
1 teaspoon salt
3 cups (6 ounces) miniature marshmallows
1 1/2 cups (9 ounces) chocolate chips
Preheat oven to 425º.
Lightly grease a half-sheet (18x13-inch) pan, or two 9x13-inch pans.
Place the oats on the prepared pan(s).
Bake for 6 to 8 minutes, stirring frequently to prevent edges from burning.
They won't brown, but should begin to smell toasty.
Remove the pan from the oven.
Transfer the oats to a bowl and add the cereal.
In a large saucepan, combine the sugar, butter, water, corn syrup, and salt.
Bring the mixture to a boil and boil for 5 minutes, or until the temperature reaches 250º on a candy thermometer.
Remove the syrup from the heat and pour it over the oats and cereal, tossing to combine.
Add 1 1/2 cups of the marshmallows and stir again until well combined.
Place the mixture on the prepared pan, patting it flat.
Press the mixture into the pan; using the lightly greased back of another half-sheet pan works well.
Sprinkle with the remaining 1 1/2 cups marshmallows and the chocolate chips.
Bake the bars for 4 to 5 minutes, until the marshmallows have puffed an are slightly brown. Remove the bars from the oven and cool for about 10 minutes on a rack.
Cut into squares while still warm.
The easiest way to do this is to use a baker's bench knife to cut the bars into long strips while still in the pan, then transfer each strip to a cutting board to cut into bars.