Preheat oven to 350°F (175°C). Line 2 baking sheets with parchment.
Cream butter, granulated sugar, and brown sugar until light and fluffy, 2-3 minutes.
Beat in egg yolk, pumpkin puree, and vanilla until smooth. If batter looks very wet, blot pumpkin with paper towels and mix again.
Whisk flour, baking soda, baking powder, pumpkin pie spice, salt, and cornstarch (if using). Add to wet ingredients and mix just until combined.
Fold in chocolate chips, 3/4 cup mini marshmallows, and graham pieces.
Scoop 2-tablespoon portions onto sheets, spacing 2 inches apart. Press a few extra marshmallows and chocolate on top.
Bake 10–12 minutes until edges are set and centers look slightly underdone; they’ll finish setting as they cool.
Cool on the pan 10 minutes, then move to a rack. Enjoy warm, gooey centers for true s’mores vibes.