Preheat oven to 375 degrees
Take a 15x10x1-inch jelly roll pan lightly, line with parchment paper, or a silicone mat.
In a bowl, sift the cake flour, baking powder, salt, and cocoa powder, together.
Beat the eggs in small bowl with an electric mixer. Beat in sugar gradually, and mix until the eggs have doubled in size.
Blend in water to the egg mixture and 1 tsp vanilla at low speed.
Slowly fold the flour mixture into the egg mixture. Once it is combined, pour into the lined pan. gently spread the batter so it evenly covers the pan.
Bake for 10-12 minutes
While it is baking, prepare a tea towel by laying it flat on a table and dust with powdered sugar.
When the cake is done baking and still hot, flip it onto the tea towel and carefully remove the parchment. Use the tea towel to roll the cake into a log from the short end.
Let the cake sit in a roll until it is fully cooled.