Chocolate Sponge Cake:

1. Prepare Oven and Pan:

   1. Preheat oven to 350°F (175°C).
   2. Line a 12×16 inch jelly roll pan (rimmed baking sheet) with parchment paper and dust it with powdered sugar.
2. Sift Dry Ingredients:
   1. Sift the cake flour, cocoa powder, and baking powder together into a small bowl. Set aside.
3. Separate Eggs:
   1. Separate the eggs, placing the yolks in a large mixing bowl and the whites in a separate large mixing bowl.
4. Prepare Egg Yolks:
   1. To the bowl with yolks, add ½ cup of sugar, salt, corn syrup, vanilla, and water.
   2. Place this bowl inside a slightly larger bowl filled with warm tap water.
   3. Beat the yolks on high speed until they become pale yellow, thick, and foamy.
   4. Remove the bowl of yolks from the water.
5. Combine Oil and Dry Ingredients:
   1. Microwave the vegetable oil until warm.
   2. Add the warm oil to the small bowl of sifted dry ingredients and whisk to combine.
   3. Add this chocolate-oil mixture to the egg yolk mixture and whisk gently until smooth.
6. Whip Egg Whites:
   1. In the large bowl with egg whites, beat on high speed until they become frothy, thick, and start to hold their shape.
   2. Slowly add the ⅓ cup of sugar a little at a time, mixing well in between.
   3. Continue to beat on high until stiff peaks form.
7. Combine Mixtures:
   1. Add the meringue mixture to the egg yolk mixture and gently fold together with a silicone spatula until just evenly incorporated.
8. Bake:
   1. Pour the batter into the prepared jelly roll pan and tap it gently on the counter to spread evenly.
   2. Bake for 15-20 minutes, or until a toothpick inserted in the center comes out clean.
   3. Do not overbake.
9. Prepare for Rolling:
   1. While baking, lay a clean dish towel flat on the counter and sprinkle generously with powdered sugar.
10. Roll Cake:
    1. Remove the cake from the oven and immediately flip the pan upside down onto the sugared towel.
    2. Remove the pan and parchment paper.
    3. Use the towel to gently roll the cake into a spiral and allow it to cool in this position.
    4. Once cool, unroll the cake for filling.

Marshmallow Buttercream Filling:

1. Combine Ingredients:

   1. Add the powdered sugar, vanilla, and butter to a large mixing bowl.
   2. Beat with a stand or hand mixer until smooth.
   3. Add the marshmallow fluff and mix again until completely combined.
2. Fill Cake:
   1. Gently unroll the cooled cake.
   2. Evenly spread the marshmallow buttercream filling over the top.
   3. Gently roll the cake back up.
   4. Set the cake seam side down on a wire rack placed over a baking sheet.

Dark Chocolate Ganache:

1. Heat Cream:

   1. Microwave the heavy cream until it just starts to bubble.
2. Combine Chocolate and Cream:
   1. Add the broken chocolate pieces to the warm cream.
   2. Allow the chocolate to sit for a minute or so, then stir to combine until smooth.
3. Pour Ganache:
   1. Pour the ganache over the rolled cake, using an offset spatula to ensure all sides are covered.
   2. Allow the ganache to set before serving.