Spread some butter (separate from the cake mix) on a shallow baking pan. Line pan with parchment paper. Separate and reserve the egg whites and yolks. Put water and sliced butter in a large heat resistant bowl (22cm diam. or more), and microwave uncovered for 3 minutes at 600W. Prepare to add 1/4 teaspoon of potato starch. Remove the bowl. Mix together the rest of potato starch and powdered green tea with a spatula. Put the uncovered bowl in the microwave again for 1 minute at 500W. Add beaten egg. Stir carefully, checking the condition of the batter. Add the rest of eggs gradually. Scoop the batter with the spatula; if it slowly drips from the spatula, you have the right consistency. Line the baking tray with aluminum foil. Pour the batter into a bag with a 1cm round opening and squeeze 10 separate portions onto the tray. Bake in the oven for 30 to 35 minutes at 200°C (392°F) While is still hot, remove the puffs from the foil and allow to cool. Cut the top part with a knife and squeeze in the custard cream with tea. Add the potato starch when water and butter come to a boil. Extend microwave time if the water and butter are taking longer to boil. If you were to scoop half of batter with a spatula, it should take around 5 seconds for all the batter to drip back into the bowl. If you reach this consistency before using all the eggs, feel free to stop adding eggs. Sift powdered tea over the puffs before serving. Whisk the egg whites in a bowl until they stiffen. Slowly add half of the sugar and continue whisking. Mix the rest of sugar with the egg yolk, and heat over a double boiler. When it reaches body temperature, take it off and whisk more until its consistency is similar to mayonnaise. Combine with , and whisk until the mixture is smooth. Sift the flour and powder tea to . To keep the foam intact, use a spatula to continue folding in the batter from the bottom of the bowl until the flour is mixed in completely. Add milk while mixing quickly with the spatula. Add batter in to the baking pan and spread evenly. Bake in the oven for 15 to 20 minutes at 180°C (356°F). When finished, remove the pan (with parchment paper) from the oven and allow to cool. Make the whipped cream: Put fresh cream and sugar into a bowl and whisk vigorously until it stiffens. Add in the tea and warm water mixture. Flip the cake pan upside down, take away the pan and remove the parchment paper from the cake. Flip the cake over again onto another sheet of parchment paper. Spread the cream from evenly across the surface and roll it into a coil. When rolling is finished, first wrap it in parchment paper, then wrap in plastic wrap, and place in the fridge for 30 minutes to help set the cake. Add your favorite fruits to the cream before rolling.