Place the bowl and the wire whisk attachment in the fridge for 30 minutes before making the filling. If you are using hand mixer, place the bowl and mixer in the fridge in the same way.
After 30 minutes, mix together sugar, cocoa powder, vanilla and 4 tablespoon of cream. This will form a smooth paste.
Add 3/4 cup of cream and beat using the wire whisk attachment of your stand mixer until stiff peaks form.
Place in the refrigerator while you work on the cake.
Pattern
Take a print out of the pattern that you want on the cake or draw free hand,
Line a 18 x 13 inch sheet pan with parchment paper and place the pattern sheet beneath it. Set aside.
In a bowl whisk together egg white and sugar. Then add flour, cocoa powder and butter. Mix till you get a smooth paste. You can add 1 tablespoon of water if the mixture appears too thick.
Pipe the mixture in a pastry bag fitted with a round tip. I used Wilton tip 2.
Trace the pattern on the parchment paper.
Once done, place the sheet pan in the freezer for 30 minutes.
Cake
Preheat the oven to 400 F degrees.
Separate the eggs. This is done best when eggs are cold.
In a bowl, mix together flour, baking powder and salt. Set aside.
Using a mixer whisk together the eggs yolks, water, vanilla, oil and 1/4 cup + 2 tablespoon of sugar till the mixture turns pale. This will take 6-7 minutes.
Start beating the egg whites.
Add cream of tartar once you see some foaming.
When they foam some more [see picture] add the remaining 1/4 cup of sugar.
Beat till egg whites form medium peaks [should hold shape but not point straight).
Fold in the flour mixture slowly into the egg yolk mixture till you have a smooth batter.
Now add the egg whites into the egg yolk-flour batter gradually. Add the egg whites in 3 parts with slow gentle strokes.
Take out the pan from the freezer and pour the batter over it.
Using a spatula spread the batter evenly throughout the pan. Tap the pan on the counter-top to get rid of any air bubbles.
Place the pan in the preheated oven at 400 F degrees and bake for 8-10 minutes or till it springs back on touch. Do not over-bake. Mine was done in 8 minutes.
Place a clean tea towel on your kitchen counter-top and dust it with lots of powdered sugar, around 1/4 cup.
Take out the cake from the oven and immediately flip it over the sugar coated towel.
Remove the parchment paper immediately.
Flip the cake again using a parchment paper so that the pattern faces up as you roll the cake.
Roll the cake using the towel and let it cool down for around 30 minutes.
Unroll the cake and fill it with whipped chocolate frosting that you prepared before.
Roll the cake, peeling the towel as you roll. Be careful not to break the cake here.