Preheat your oven to 350°F and line two baking sheets with parchment paper. Cover a small plate with plastic wrap. Using a teaspoon or melon baller, scoop out 8 heaping mounds of marshmallow fluff onto the plate. Pop it in the freezer for at least 10 minutes — this is key so the fluff holds its shape when you wrap the dough around it.

In the bowl of a stand mixer, cream the softened butter, brown sugar, and granulated sugar together until smooth and fluffy.

Add the egg and vanilla, mixing until well combined.

Add the cocoa powder, flour, baking soda, and salt. Mix thoroughly, scraping down the sides of the bowl as needed.

Divide the dough into 8 equal portions and flatten each one into a disc about ½ inch thick.

Take the marshmallow fluff out of the freezer and work quickly — place one frozen mound of fluff in the center of each dough disc.

Wrap the dough up and around the fluff, pinching the edges to seal it in completely. Make sure there are no gaps or the fluff will leak out during baking. Bake for approximately 10 minutes, or until the centers look set. Allow the cookies to cool completely before decorating — this is non-negotiable, or your ganache will slide right off.

To make the ganache, combine the semi-sweet chocolate chips and heavy cream in a microwave-safe bowl. Microwave in 30-second bursts, stirring in between, until the mixture is completely smooth.

Spoon a generous layer of ganache over each cooled cookie and spread it evenly to the edges.

Melt the white chocolate chips in a small bowl using the same microwave method. Transfer to a piping bag (or a zip-top bag with the corner snipped) and pipe a loop or swirl pattern over the ganache, mimicking the classic cupcake cookie look.

Let the chocolate set until slightly hardened before serving.