In a mixing bowl, beat together softened butter, granulated sugar, brown sugar, and vanilla until light and fluffy (3-5 minutes).
Add egg yolks and mix until just combined. Then, add cornstarch, flour, baking powder, baking soda, and salt. Mix until the dough comes together.
Roughly chop the milk chocolate and graham crackers. Fold in ½ cup of each into the dough. Save the rest for topping.
Halve the marshmallows. Line a baking sheet with parchment paper.
Scoop 2 tbsp of dough for each ball, flatten lightly, and place half a marshmallow in the center. Seal the dough around the marshmallow.
Place the cookies on the baking sheet. Cover and chill in the refrigerator for at least 2 hours (preferably 24 hours).
Preheat the oven to 350°F. Place chilled dough balls on a lined baking sheet, about 2 inches apart. Top each with additional chocolate and graham crackers.
Bake for 9 minutes until the edges are set and golden brown. Do not overbake!
As soon as the cookies come out, use a round cookie cutter or glass to reshape them. Let cool for 4 minutes on the sheet before transferring to a rack to cool completely.
Enjoy these Easy Oven S’mores Cookies for up to 5 days! Perfect for a sweet treat any time.