Bring milk and butter to a boil, then stir in cake flour, bread flour, and genmaicha powder to form a paste. Transfer to a mixer bowl and stir slowly with a beater. Gradually add eggs and egg yolks. Whip egg whites with sugar to medium peaks, then fold into genmaicha mixture. Spread 900g–1kg of batter into a tray. Bake at 200–210°C for 8–10 minutes.