In a medium bowl, whisk together the all-purpose flour, baking soda, baking powder, corn starch, and kosher salt. Set aside.
In a large mixing bowl (or the bowl of a stand mixer), cream the softened butter and granulated sugar together for 2 minutes on medium speed until the mixture is light, fluffy, and pale in color.
Add the egg, egg yolk, and vanilla extract. Mix until they are fully combined. Make sure to scrape down the sides of the bowl as needed.
Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined.
Gently fold in the Lucky Charms marshmallows using a spatula or pulsing the stand mixer on the lowest speed. If you are using confetti sprinkles, fold them in now as well.
Using a 2 inch cookie scoop (about 2-2.5 tbsps) scoop the dough onto a large baking sheet. Wrap the cookies in plastic wrap, and refrigerate for a minimum of 30 minutes.
While the dough is chilling, preheat the oven to 350 degrees. Line a large baking sheet with parchment paper or a silicon baking mat. Arrange six cookies on the baking sheet around two inches apart.
Bake for 10-12 minutes, or until the edges are set. Leave the cookies on the hot baking sheet for 5 minutes to continue cooking. (Optional) For perfectly round cookies, use a round cookie cutter or glass larger than the cookie, and gently swirl it around the cookie edges until you get the desired shape.
Immediately after shaping and while the cookies are still warm, press 3 to 4 extra marshmallows into the top of the cookie, pressing deep into the cookie so they don't fall off once cooled.
After the cookies have sat on the baking sheet for 5 minutes, transfer the cookies to a cooling rack to cool completely.