Lucky Charms Frosted Flakes cookies are soft and chewy with a hint of crunch and a smattering of sweet thanks to those crispy flakes and colorful marshmallows. NOTE: This dough requires chilling.
Cream butter and sugars together in large bowl. Add oil; mix until smooth. Add egg and vanilla; mix until thoroughly combined. Stir in baking soda and salt.
Add oats and flour and mix until just combined. Stir in Lucky Charms Frosted Flakes cereal. Note: The mixer will crush the flakes and crumble the marshmallows. We like the cookies that way but if you want larger marshmallows, i.e., whole pieces, gently fold them in.
Use a medium cookie scoop to scoop balls of cookie dough and place them on the baking sheet. (You can space them close together because you'll be baking them on a separate cookie sheet.) Cover with plastic wrap; chill at least one hour.
Preheat oven to 350 degrees. Line a separate baking sheet with parchment paper or silicone baking mat.
Remove cookie dough from fridge and let sit 5 minutes before baking. Place cookies 2 inches apart on baking sheet. Bake 10 to 12 minutes or until edges just barely start to turn golden brown. Cool 2 minutes before removing to a wire rack. Store in an airtight container up to 5 days.