Preheat the oven to 350 degrees F. Line two baking sheets with parchment paper and set aside.

Cover a plate with plastic wrap. Using a teaspoon, melon baller, or similar scoop, scoop the marshmallow fluff into 8 heaping mounds on the prepared plate. Freeze until ready to use, at least 10 minutes.

For the cookie base, combine the softened butter, brown sugar, and granulated sugar in the bowl of a stand mixer. Cream until smooth.

Add the egg and vanilla and mix well.

Add the cocoa powder, flour, baking soda, and salt. Mix thoroughly, scraping down the sides often.

Divide the dough into 8 equal portions. Flatten each portion into a disc about 1/2 inch thick.

Remove the marshmallow fluff from the freezer. Working quickly, place one fluff mound in the center of each cookie dough disc.

Wrap the dough around the fluff and seal it in completely.

Bake the cookies for about 10 minutes, or until the centers are set. Allow the cookies to cool completely before decorating.

For the topping, combine the semi-sweet chocolate chips and heavy cream in a medium bowl. Melt in the microwave in 30-second intervals, stirring between each one, until smooth.

Spread a spoonful of ganache over the top of each cookie.

Melt the white chocolate chips in a small bowl until smooth.

Transfer the melted white chocolate to a piping bag.

Pipe the white chocolate over the ganache in a loop pattern similar to chocolate mallow cupcakes. Allow the topping to set until slightly hardened.