In a large mixing bowl using an electric mixer combine the butter and sugar until light and fluffy (about 2-3 minutes).
Add the vanilla extract and eggs and mix for another 2-3 minutes.
Add the flour, cocoa, baking soda and water. Mix until just combined.
Scoop out 18 balls of the cookie dough, placing them 2 inches apart on a baking sheet that has been lined with parchment paper or a silicone baking sheet.
Bake for 12-15 minutes until the cookies are just cooked through.
Let the cookies cool for a few minutes on the cookie sheet and then transfer to a wire rack to cool completely.
Make the buttercream frosting
In a medium bowl using an electric mixer combine the softened butter with the marshmallow cream until combined.
Mix in the vanilla extract and the the powdered sugar 1 cup at a time, mixing until completely smooth.
Fill a piping bag or ziploc bag with the tip cut off. Pipe the marshmallow buttercream onto the center of the completely cooled 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. Heat in 30 second intervals, stirring between each interval until the ganache is completely melted and smooth.
Spoon the ganache over the marshmallow buttercream.
White chocolate drizzle
In a small microwave safe bowl, melt the white chocolate in 30 second intervals, stirring between each interval until completely smooth. Be careful not to heat any more than necessary or the chocolate will seize.
Place the melted white chocolate into a small piping bag or ziploc bag with the tip cut off.
Pipe the classic squiggle on the top of the cookies.