Preheat the oven to 350 degrees Fahrenheit and line 2-3 baking sheet pans with parchment paper. Set aside.
Add 1 (15.25-ounce) dark chocolate fudge cake mix, 2 tablespoons unsweetened cocoa powder, ½ cup vegetable oil, and 2 large eggs to a large bowl and mix with a hand mixer or stand mixer fitted with the paddle attachment until a thick batter forms.
Use a medium cookie scoop (about 1½ tablespoons) to scoop the dough, then roll it into uniform balls with your hands. Place them on the prepared baking sheets, leaving about 2 inches apart.
Bake for 10-12 minutes, until the edges are set. The center of the cookies will look underbaked, but they will continue to cook as they cool. Let them sit on the baking pan for 2 minutes before transferring them to a cooling rack to finish cooling.
Marshmallow Cream
Add ½ cup softened unsalted butter, 2 cups powdered sugar, and 1 pinch salt to a large bowl and beat with a hand mixer or stand mixer fitted with the paddle attachment until combined. It will be crumbly at this stage.
Add 1 (7-ounce) container marshmallow creme and ½ teaspoon vanilla extract to the cream and beat until combined, scraping down the sides and bottom of the bowl to ensure everything is incorporated.
Spoon the cream into the center of each cooled cookie. Spread it into an even circle, leaving some of the cookie exposed. Don't frost the whole surface. Let the cookies sit for at least 10 minutes before topping with ganache.
Ganache and Swirl
Make the ganache by adding ½ cup milk chocolate chips and 3 tablespoons heavy whipping cream to a small microwavable bowl. Microwave for 30-45 seconds and stir until smooth. The ganache will seem runny. Keep stirring and heating in 10-second increments until smooth and melted.
Spoon a heaping teaspoon of ganache in the center of the marshmallow cream and spread it out into a circular layer. Allow it to set up for a few minutes.
Let the ganache cool completely and set before heating ¼ cup white candy coating chocolate in a small bowl in the microwave in 20-second increments until fully melted. Add the melted chocolate to a small ziplock bag with the corner barely snipped off. You can also use a piping bag fitted with a small round piping tip (size #1 or #2). Pipe the signature swirls over the top of the ganache.
Notes
Storage Instructions
Storage: Keep cookies in an airtight container for up to 3 days at room temperature or 5 days in the fridge.
Freezing: Don't freeze the finished cookies. The marshmallow and chocolate can turn sticky as they thaw. Instead, freeze the plain cookie base for up to 3 months, then thaw completely before adding the toppings.