Combine the ground flaxseed and water in a small bowl to form the flax egg.

Sift together the flour, cereal, baking soda and sea salt in a mixing bowl.

In another bowl, use an electric mixer to combine the butter and both sugars for 2 minutes. Add in the vanilla extract and flax egg and mix for another 30 seconds.

Combine the wet and dry mixtures in 2 batches with a wooden spoon. Fold in the marshmallows and chocolate chips.

Place the batter in the fridge for 20 minutes.

Meanwhile, preheat the oven to 350 degrees. Lightly coat a large baking sheet with cooking spray.

Scoop the batter onto the baking sheet, forming about two dozen cookies. Bake for 9-12 minutes, until they're lightly browned on the bottoms.

Allow the cookies to cool on the baking sheet for a few minutes before transferring to a cooling rack. Enjoy!