Prep. Preheat your oven to 350 degrees. Line a cookie sheet with parchment paper an set aside.
Prepare the Cookie Dough. In a large mixing bowl, combine the butter and sugar using an electric mixer until light and fluffy. Add the vanilla extract and eggs, then mix to combine. Add in the dry ingredients; flour, cocoa, baking soda, and water, mixing until just combined.
Bake the Cookies. Scoop out 12 balls of the cookie dough, placing them 2 inches apart on the lined cookie sheet (you may need to do this in batches). Bake for 12-15 minutes, until the cookies are just cooked through. Set aside to cool completely.
Make the Marshmallow Cream. In a medium bowl, combine the softened butter and marshmallow cream with an electric mixer until well combined. Mix in the vanilla extract, then gradually add the powdered sugar, 1 cup at a time.
Apply the Marshmallow Cream. Fill a piping bag or a Ziploc bag with the tip cut off with the prepared marshmallow cream. Pipe the marshmallow frosting onto the center of the cookies. Place the cookies in the fridge while you prepare the ganache.
Make the Ganache. In a microwave-safe bowl, combine the chocolate chips, butter, milk, and corn syrup. Microwave in 30-second intervals, stirring between each, until the mixture is melted and smooth.
Apply Ganache to Cookies. Spoon the ganache over the marshmallow buttercream-topped cookies.
Add the White Chocolate Detail. In a small microwave-safe bowl, melt the white chocolate in 30-second intervals. Transfer the melted white chocolate into a small piping bag or a Ziploc bag with the tip cut off. Pipe the classic squiggle pattern on top of the cookies. Allow the cookies to set and enjoy!