Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper and set aside.

In the bowl of a stand mixer fitted with the paddle attachment (or using an electric hand mixer), cream together the softened butter and granulated sugar on medium speed until light and creamy, about 2-3 minutes.

Scrape down the bottom and sides of the bowl. Add the slightly chilled egg, mix on medium speed for 20 seconds, then add the cold egg yolk. Scrape down the sides, add vanilla extract, and mix on low until combined.

Add the flour, Dutch-processed cocoa powder, baking soda, and salt. Mix on low speed until the dry ingredients are just incorporated. Be careful not to overmix.

Using a 3-tablespoon cookie scoop, form 12 cookie dough balls. Take two dough balls and roll them together to create one large cookie ball. You should end up with 6 large cookies.

Place the cookies on the prepared baking sheet, spacing them evenly.

Bake in the preheated oven for 10-12 minutes. When the cookies are still warm, Gently flatten each to about ½ inch in thickness. Allow the cookies to cool on the baking sheet for 10 minutes before transferring them to a cooling rack. While the cookies are cooling, make the marshmallow filling.

In a stand size mixer fitted with a paddle attachment or in a bowl with a hand mixer, beat the butter, marshmallow fluff, powder sugar, and heavy cream medium speed until smooth and fluffy!

Add the vanilla extract, and salt. Mix on medium-high speed for 2–3 minutes, or until the filling is light, fluffy, and spreadable.

If the filling feels too soft to pipe or spread, chill it in the fridge for 10–15 minutes to firm it up slightly. You don’t want it going over the edge of the cookie.

When the cookies are completely cooled, use a large piping tip or a large hole from a pipping bag, pipe the frosting in the center of the cookie and drizzle melted chocolate on top!