Start by preheating your oven to 425°F. This step is crucial as it prepares the environment for baking the oats. Lightly grease a half-sheet (18x13-inch) pan or two 9x13-inch pans to prevent sticking. Make sure to cover all the surfaces!
Next, place the rolled oats on the prepared pan(s). Spread them out evenly so they toast nicely. Bake for about 3 minutes, then stir the oats and return them to the oven for an additional 3 minutes. You’ll know they’re ready when they start to smell toasty but haven’t browned too much.
Once the oats are toasted, remove them from the oven and transfer them to a bowl. Now, add in the Rice Krispies, mixing them together well. It’s important to keep the oven on for later!
In a large saucepan over medium heat, combine the granulated sugar, unsalted butter, water, light corn syrup, and salt. Stir until the mixture comes to a boil. Once boiling, let it cook for about 5 minutes until it reaches 250°F on a candy thermometer. Be attentive to prevent burning!
Remove the saucepan from heat and pour the boiling mixture over the oats and cereal in the bowl. Toss everything together until well combined. You should see a gooey mixture forming.
Add 1½ cups of the miniature marshmallows into the mixture. Stir again until they’re evenly distributed throughout. You’ll get a nice blend of gooey marshmallow and crunchy oats.
Now, place the mixture back onto the prepared pan that you used for the oats, patting it down flat with your hands or a spatula. This step helps to compact the bars. Sprinkle the remaining 1½ cups of miniature marshmallows and chocolate chips on top. If you have another greased pan, you can press down on the mixture to make it even.
Return the pan to the oven for about 3 minutes. This will puff up the marshmallows slightly and give them a light golden color. Keep a close watch to avoid burning!
Once done, remove the pan from the oven and let it cool on a rack for about 10 minutes. Cutting them while warm is easier, so grab a baker's bench knife or a knife with a wide blade to slice them into squares. If the mixture is too gooey, allow it to cool a bit more before cutting.