1. Make the Chocolate Cookies:
Preheat your oven to 365°F (185°C). Line a baking sheet with parchment paper and set aside.
Using a stand mixer fitted with a paddle attachment, cream together the butter, brown sugar, and granulated sugar on medium speed for 1-2 minutes until light and fluffy.
Add the eggs, light corn syrup, and vanilla extract, mixing until combined. Scrape down the sides of the bowl with a rubber spatula as needed.
Add the flour, cocoa powder, cornstarch, baking soda, and salt to the wet ingredients. Mix on low speed until just combined.
Portion the dough into ¼ cup-sized balls and place them on the prepared baking sheet, spacing them out to allow room for spreading. Using the palm of your hand, gently press down on each ball to create a thick disk.
Bake for 10 minutes, or until the edges are set and slight cracks appear around the edges. Do not overbake.
Remove from the oven and let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.
2. Prepare the Marshmallow Meringue Frosting:
Fill a small pot with 1-2 inches of water and bring it to a simmer. In a large heatproof mixing bowl, combine the granulated sugar and egg whites. Place the bowl over the simmering water, ensuring the bottom of the bowl doesn’t touch the water.
Whisk constantly until the mixture reaches 160°F (71°C) on an instant-read thermometer and the sugar has dissolved.
Transfer the mixture to the bowl of a stand mixer fitted with a whisk attachment. Beat on medium speed until stiff peaks form, about 4-5 minutes.
Add the marshmallow fluff and vanilla extract, then mix on high speed until stiff peaks form again, another 4-5 minutes. Transfer the frosting to a piping bag.
3. Make the Ganache & Topping:
Place the semi-sweet chocolate in a heatproof bowl. In a small pot, heat the heavy cream over medium heat until it comes to a boil. Pour the hot cream over the chocolate and let it sit for 2 minutes.
Slowly whisk the chocolate and cream together starting from the center, until smooth. Let the ganache sit for a few minutes to thicken slightly before using.
Melt the white chocolate wafers (or chips with vegetable oil) in a microwave-safe bowl in 30-second increments, stirring in between until melted. Transfer the melted white chocolate to a small Ziploc bag and snip a small corner off to create a piping bag.
4. Assemble the Cookies:
Pipe the marshmallow frosting onto the top of each cooled cookie.
Add a dollop of chocolate ganache on top of the frosting and swirl gently to cover part of the frosting.
Pipe the signature Hostess cupcake-style loopy white circles across the center of each cookie using the melted white chocolate.