Preheat oven to 400 degrees F. Line two baking sheets with parchment paper.
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugars together for 3-4 minutes until creamy and light. Add 1 cup of the peanut butter, egg, egg yolk, and vanilla, then beat again, scraping down the bottom and sides of the bowl.
Add both types of flour, cornstarch, baking powder, baking soda, and salt, mixing just until combined.
Divide the dough into 9-12 large balls. You can weigh them to be exact, use a large cookie scoop, or just pinch off portions and eyeball it.
Make an indentation in the center of each ball of dough with your thumbs and place a large marshmallow inside. Wrap the dough around the marshmallow, although it's okay if a bit of the marshmallow peaks out.
Space the cookies apart on the baking sheets so they have room to spread. Bake for 13-15 minutes until done. They should be just barely starting to turn golden brown and set around the edges. Be careful not to overbake or the marshmallow inside will melt away and the cookie won't be as soft. Immediately after removing the cookies from the oven while they are still hot, use two spoons to shape the cookies back into circles if they got a little misshapen while baking.
Let the cookies cool on the baking sheet for at least 15 minutes before transferring to a wire rack.
Top each cookie with a spoonful of marshmallow fluff, then drizzle with about ¼ cup of additional peanut butter that has been warmed in the microwave for 20 seconds and transferred to a ziploc bag with the corner snipped off.