Kasutera is a traditional Japanese sponge cake made of sugar, flour, eggs, and honey. There are now many varieties made with ingredients such as matcha green tea, cocoa or brown sugar.
Line a 44x12x6-cm wood box or an 9-inch baking pan with parchment paper. Fill half of a large pot with water and bring it to a boil. Stir together the honey and green tea drink in a small bowl. Combine the flour, matcha tea powder and sift twice and set aside. Preheat the oven to 175C/350F.
Place the eggs and the sugar into the bowl of your mixer. Set the bowl over the pot of hot water and whisk the mixture until lukewarm and the sugar is dissolved. Remove and beat over medium speed until the mixture starts to thicken. Lower the speed and continue whisking until it is thick and smooth. When you lift the whisk, the peaks drops slightly. Beat in honey water mixture in a few additions until incorporated.
Sift in the flour in a few additions and whisk on low speed until fully incorporated. Pour the batter into the prepared wood box or the pan. Place in the hot oven and bake until the cake is golden brown and it feels spongy not tacky to the touch, about 50 minutes. Cool on the rack for 10 minutes, and remove from the pan.