Preheat the oven to 350°F (180°C) and line a baking sheet with parchment paper. Set aside.

In a large bowl or in the bowl of a stand mixer fitted with the paddle attachment, cream together the softened unsalted butter and granulated sugar until light and fluffy, about 2–3 minutes.

Scrape down the sides and bottom of the bowl as needed. One at a time, add the egg yolk and mix on medium speed for 20 seconds, scrape down the sides and bottom of the bowl and add the whole egg. Mix again for 20 seconds. Add the red velvet emulsion (or red food coloring) and vanilla extract and mix until fully combined.

Add the Dutch process cocoa powder, all-purpose flour, graham cracker crumbs, salt, cornstarch, and baking powder and mix on low speed until just combined and a soft dough forms.

Using a 3 tablespoon cookie scoop, portion the dough into 15 equal scoops. Take 5 of the scoops and divide each one in half, creating 10 smaller pieces of dough. Set aside. Take the remaining 10 full scoops and gently flatten each into a thick disc, about 1 inch thick. Spoon 1–2 teaspoons of marshmallow fluff into the center of each disc. Place one piece of the smaller dough over the marshmallow fluff and gently press the seams together to seal. It is okay if some of the marshmallow peeks through the sides. You should end up with 10 cookies total.

Place the cookies on the prepared baking sheet, spacing them a few inches apart. Bake for 9-11 minutes, or until the edges are set and the centers are still slightly underbaked.

Immediately after baking, press a piece of chocolate bar onto the top of each warm cookie and sprinkle with additional graham cracker crumbs, if desired. Allow the cookies to cool on the baking sheet for 10 minutes before transferring to a wire rack.