1.In a bowl of a stand mixer fitted with the paddle attachment, cream together butter and sugars until light and fluffy. Add egg, egg yolk, and vanilla extract and beat together until well-combined.
2.In a separate, medium-sized mixing bowl whisk together flour, baking powder, baking soda, and salt. Gradually stir dry ingredients into wet ingredients until completely combined.
3.Add rice krispie cereal and mini marshmallows and use a spatula to stir until evenly distributed throughout the dough. Preheat oven to 350 degrees. Line a large baking sheet with parchment paper, set aside.
4.Scoop cookies by 3 tablespoon-sized scoops or 54 grams and gently roll between your palms to form a smooth ball. Place on baking sheet, spacing cookies at least 2” apart.
5.Transfer to center rack of oven and bake for 13-14 minutes. Cookies are finished baking when the edges of the cookies are beginning to turn light golden brown.
6.Immediately after removing from the oven, if your cookies are messy you can use the back of a spoon to help round the cookies and push any melted marshmallow back in. Allow cookies to cool completely on baking sheet before enjoying!