Steps
1. Preheat oven to 350°F (175°C).
2. In a bowl, mix devil’s food cake mix, egg white, water, and corn syrup until thick batter forms.
3. Scoop 1 tbsp-sized rounds onto a parchment-lined baking sheet.
4. Bake for 8-10 minutes, avoiding overbaking. Let cool.
5. For filling, beat together marshmallow fluff, powdered sugar, vanilla extract, and add water if needed for spreading consistency.
6. Spread or pipe filling onto the curved side of half the cookies. Top with remaining cookies to make sandwiches.
7. Refrigerate assembled cookies for 15-20 minutes.
8. Whisk together powdered sugar, cocoa powder, corn syrup, meringue powder, vanilla extract, and water to make chocolate coating.
9. Dip each sandwich cookie fully into the coating, let excess drip off.
10. Place dipped cookies on parchment paper and chill until coating is fully set.