Preheat the oven to 400ºF. Spray a 17x12-inch baking pan with nonstick cooking spray, then line with parchment paper. Spray the parchment paper with the nonstick cooking spray.

In a small bowl, whisk together the flour, cornstarch, cocoa powder, and salt.

Add the egg yolks, the sugar, and the vanilla to the bowl of a stand mixer. Beat on high until the mixture increases in volume and is thick and pale, about 5 minutes.

Remove the bowl from the mixer. Sift half of the dry ingredients into the bowl and fold in gently with a spatula. Once incorporated, sift in the second half and fold in.

In another bowl, beat the egg whites until they form stiff peaks.

Gently fold the egg whites into the cake batter, only stirring until no white streaks remain.

Pour the cake into the prepared baking pan. Tap the pan on the counter a few times to remove any large air pockets.

Bake in the preheated oven just until the cake is set, about 8-10 minutes. (Do not overbake the cake or it will crack when you roll it.)

Starting on the short end, roll the cake gently. You don’t need to roll it to tight. Let the cake sit until room temperature, about 30 minutes to an hour.

Add the cream to a mixer bowl and whip until thick. Sift the sugar into the cream and add the vanilla. Continue to beat until soft peaks form.

Melt the marshmallow fluff in the microwave until just melted. Stir and let it cool slightly.

Add the marshmallow fluff to the whipped cream and continue to beat until you have stiff peaks.

Once the cake is cool, gently unroll it. Spread the whipped cream filling over the cake, then re-roll, rolling gently to keep the filling from squishing out. Wrap the cake with plastic wrap and refrigerate for at least an hour, but preferably overnight.

Place the chocolate in a medium sized bowl.

In the microwave or in a saucepan on the stove, heat the cream until just before boiling. Pour over the chocolate and let it sit for about a minute, then stir until the chocolate is melted and the mixture is smooth. Let the mixture cool until room temperature.

Transfer the chocolate to a mixer bowl and whip until it is thick.

Remove the cake from the refrigerator and remove the plastic wrap. Cut about 1/4 to 1/3 of the cake on a diagonal from one of the end. Place the larger portion on a serving plate, then place the cut end of the smaller section along the side of the larger section.

Spread the ganache over the top, leaving the ends exposed to show the swirl. You can run a spatula down the sides to resemble bark, or run a fork across the cake to make a bark texture.

Garnish the cake and serve.