
Preheat oven to 350F (175C). Line baking sheet with parchment paper, set aside.
In a large mixing bowl (or the bowl of a stand mixer fitted with the paddle attachment), use an electric mixer to cream together butter and sugars until light and fluffy.
Add egg, egg yolk, and vanilla extract and beat together until well-combined.
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.
Add rice krispie cereal and mini marshmallows and use a spatula to stir until evenly distributed throughout the dough.
Scoop cookies by 3 Tablespoon-sized scoop (54g) (I do 2 level scoops from my cookie scoop, linked above, and roll them together) and gently roll between your palms to form a smooth ball. Place on baking sheet, spacing cookies at least 2” (5cm) apart.
Transfer to center rack of 350F (175C) oven and bake for 13-14 minutes. Cookies are finished baking when the edges of the cookies (not any melted marshmallow!) are beginning to turn light golden brown.
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 (an optional step that neatens them up but isn’t mandatory!)..
Allow cookies to cool completely on baking sheet before enjoying!