In the bowl of a stand mixer, combine melted butter, granulated sugar, light brown sugar, and vanilla extract. Mix on medium for 1 minute.

Add eggs and mix on low until incorporated.

Add flour, corn starch, cocoa powder, baking soda, and salt. Mix on low until combined.

Line a baking sheet with parchment paper. Scoop about 3 tablespoons of dough per cookie and place on the prepared sheet. Chill the dough for 30 minutes to enhance flavor and prevent spreading. Preheat the oven to 350°F.

Once chilled, space cookies about 2 inches apart on a parchment-lined baking sheet. Bake for 14 minutes, or until edges are just set. Let cool on the sheet for 3 minutes before transferring to a wire rack to cool completely.

In a stand mixer, beat together butter, marshmallow fluff, and vanilla on medium speed until combined.

Add powdered sugar, salt, and 2 tablespoons of milk or heavy cream. Mix until smooth. If the mixture is too thick, add more milk or cream until it reaches a thick, spreadable consistency.

While cookies are cooling, make the ganache. Break the chocolate into pieces (if not using chocolate chips) and place in a heatproof bowl.

Heat the cream in a small pot over medium heat until it starts to simmer. Pour over the chocolate and let sit for 5 minutes. Whisk until smooth. Let cool for 15 minutes until it thickens and is spreadable.

Put about 1/4 cup of the marshmallow cream in a piping bag with a small round tip and set aside.

Once the cookies are completely cool, assemble them. Scoop about 3 tablespoons of marshmallow cream onto each cookie and spread, leaving about 1/4 inch around the edges.

Chill the cookies between layers for 30 minutes (optional). Spread about 2 tablespoons of ganache over the marshmallow cream, again leaving about 1/4 inch around the edges.

Pipe swirls on top of the cookies and serve. Refrigerate to allow the ganache to set, or enjoy at room temperature for a softer ganache.