Cooking spray
All-purpose flour, for pan
1 (15.25-oz.) box Devil's food cake mix, plus ingredients called for on box
2 (7.5-oz.) jars Marshmallow Fluff
1 c. (2 sticks) unsalted butter, softened
2 tsp. kosher salt
2 tsp. pure vanilla extract
3 c. (345 g.) (or more) confectioners' sugar, divided
1 1/2 c. semisweet chocolate chips
3/4 c. heavy cream
1 tbsp. (or more) whole milk
Step 1
Preheat oven to 350°. Generously grease a 17" x 12" half-sheet or jelly roll pan with cooking spray; lightly sprinkle with flour, tapping out excess.
Step 2
Prepare cake batter according to box directions. Spread batter into prepared pan; smooth into a thin, even layer with a rubber spatula.
Step 3
Bake cake until a tester inserted into the center comes out clean, 22 to 25 minutes. Let cool in pan on a wire rack.
Step 4
Meanwhile, in the large bowl of a stand mixer fitted with the whisk attachment, beat Fluff and butter until smooth and fluffy. Add salt, vanilla, and 2 cups confectioners' sugar and beat until smooth.
Step 5
Using an offset spatula, spread marshmallow frosting on cooled cake.
Step 6
Place chocolate chips in a medium heatproof bowl. In a small saucepan over medium heat, heat cream until bubbles begin to break the surface around edges of pan. Pour hot cream over chips, swirling bowl to coat chips. Cover bowl and let sit 3 to 5 minutes. Uncover and, starting from the center and working your way to edges of bowl, gently whisk until chocolate is melted and smooth. Let cool to room temperature.
Step 7
Pour cooled ganache over marshmallow frosting; spread with offset spatula. Refrigerate until set, about 10 minutes.
Step 8
In a medium bowl, whisk milk and remaining 1 cup confectioners' sugar until smooth. If icing is too loose, add more confectioners' sugar; if it’s too thick, add more milk.
Step 9
Transfer icing to a pastry bag fitted with a small tip. Pipe decorative doodle patterns on a diagonal across ganache.