- Separate the eggs and put the whites in the fridge. Allow the yolks to reach room temperature. Preheat oven to 190°C (375°F).
- Warm up the milk. It needs to be warm to the touch, not boiling hot.
- Mix the flour, potato starch, baking powder and matcha thoroughly, and sift them together three times to remove any lumps.
- In a bowl, break the yolks, add half the sugar and whisk until they reach about double the size and change their colour to almost white. Wash your whisks and begin whisking the whites until they become slightly foamy. Gradually add the remaining sugar over three separate times and beat until stiff peaks form and egg whites are glossy.
- Once the egg whites are compact and creamy, take 1/3 of them and add to the yolks and sugar mixture, and gently mix with a large spoon. From now, on stop using electric whisks. Move the yolk mixture back into the egg whites bowl and mix with a large spoon. Mix slowly with wide movements. Try keeping the same orientation when you scoop (i.e. clockwise).
- Add the dry ingredients (the mixture of flour, starch, baking powder and matcha) all at once, and keep scooping gently until it’s fully incorporated. Finally, add the warm milk and mix gently until incorporated.
- Line a 38 cm x 25 cm (15” x 10”) jelly roll pan with parchment paper. Pour the batter into the baking pan and spread it evenly. Bake in preheated oven for 10 to 12 minutes, depending on your appliance. You can check if it’s ready by piercing it with a toothpick. It should stay dry. Don’t over bake or it will break when rolling.
- As soon as you take it out from the oven, place another parchment paper sheet over it, turn it upside down, and remove the pan. Using a knife or a spatula, gently separate the top sheet from the cake. Slice diagonally 1 cm from one of the sides.
- While the cake is still hot, gently start rolling the cake, keeping the paper attached to it. (This will allow it to retain the bend while cooling down, and the paper will prevent it from sticking.) Cover the roll with a kitchen towel and allow it to cool to room temperature. (About half an hour.)
- Prepare the filling. Combine the cream, sugar and matcha in a bowl. Whisk until firm and spreadable.
- Transfer the cake to a flat surface and unroll. Pour the filling over the cake and spread it. Leave a 1 to 1 1/2 cm (about 1/2 inch) border on all sides, and spread it so that the layer becomes thinner as it approaches the sliced side.
- Start rolling from the non-sliced side. Don’t be afraid to put some pressure on it, but avoid breaking the cake. Once the cake is fully rolled, use the parchment paper to wrap it and twist the sides like a candy wrapper. Put it in the fridge and let it cool for at least 2 hours.
- Remove from the fridge and unwrap. Cut away 1 or 2 cm (about 3/4 inch) of cake from the ends and transfer to your serving dish!