Pre-heat oven to 350 degrees.
In the bowl of a stand mixer or in a large mixing bowl with hand mixer, beat together butter and both sugars until light and fluffy, 2-3 minutes.
Mix in eggs and vanilla until combined.
In a separate medium bowl, whisk together both flours, cocoa powder, baking soda, and salt.
Mix dry ingredients into wet ingredients until just combined.
Fold in mini marshmallows and mini M&Ms.
Lightly spray a 10-inch cast iron skillet with cooking spray. Spread cookie dough in skillet and lightly press into an even layer.
Bake at 350 degrees for 21-24 minutes, until a toothpick inserted in middle of cookie comes out clean.