Prepare the Batter
Beat the egg yolks and sugar (for egg yolks) until smooth.
Add vegetable oil and water, and beat until smooth.
Mix in the sifted cake flour and matcha powder until combined, then set aside.
In another bowl, beat the egg whites until foamy.
Add half of the sugar (for egg whites) and continue beating until slightly stiff.
Add the remaining sugar (for egg whites) and beat until stiff peaks form.
Gently fold the beaten egg whites into the egg yolk mixture in about three additions until well combined.

Bake the Roll Cake
Preheat the oven to 350°F. Line a baking sheet with parchment paper.
Pour the batter onto the parchment paper, spreading it evenly and removing any air bubbles.
Bake for 15 minutes, then let it cool.

Make the Matcha Cream Filling
Mix the matcha powder and sugar with hot water until smooth, then let it cool.
Place the heavy cream over ice water and beat until stiff peaks form, gradually adding the matcha mixture.
Refrigerate the whipped cream until ready to use.