Preheat oven to 350°. Grease and line a 9×13 metal baking pan with parchment paper that overlaps the sides.

In the bowl of a stand mixer or in a large bowl with a handheld mixer, add flour, sugar, cocoa, baking soda, baking powder, and salt and combine.

Add milk, vegetable oil, eggs, and vanilla extract to flour mixture. Mix on medium speed until well combined.

Reduce speed and carefully add coffee (or boiling water) to the cake batter until well combined.

Pour into the prepared baking pan and bake for 35-45 minutes or until baked through. Check with a toothpick.

Remove from oven and allow to cool for 30 minutes at room temperature before refrigerating until completely chilled, at least two hours or overnight.

Prepare filling and ganache while cake bakes. In a medium bowl prepare the filling. Beat butter and a little more than ¾ of the marshmallow fluff until smooth.

Add vanilla extract, powdered sugar, salt and milk. Add more milk if needed to reach a soft consistency. Set aside.

For the chocolate Ganache, add chopped chocolate to a medium heat-proof bowl and heat the cream in a small saucepan over medium heat until it begins to gently simmer.

Pour the cream over chocolate, and let sit for 2–3 minutes until melted. Use a spatula to mix until smooth. Refrigerate for two hours until thick.

Once the cake has completely cooled, lift the cake out of the pan and use a serrated knife to slice the cake in half lengthwise.

Place the bottom half of the sheet cake down and add the marshmallow cremé filling, reserving about a ½ cup for piping the squiggle.

Evenly spread over the center of the cake and top with the other half of the cake.

Evenly spread the chocolate ganache on the top of the cake and refrigerate for 1 hour.

Add extra marshmallow cremé filling to a piping bag fitted with a small round tip and pipe 5 small looped lines from one side to the other widthwise.

Slice and serve. Store in the refrigerator.