Add 3 eggs, 1 egg yolk, sugar mix 140g and corn syrup 60g into a kneading bowl and mix them with a hand mixer on high speed for 5 to 6 mins.
Add the Flour mix 100g into the bowl above (step 1) and quickly mix it in with the spatula. (If you mix it for too long, the Castella can get chewy, so watch out.)
Add the milk and mix well.
Pour the dough into the cake tray (cover up 2/3 of the tray) and bang the cake tray on the board lightly a couple of times to remove air bubbles.
Put the cake tray in the bottom of the pre-heated oven and bake it at 150C to 170C for 30 to 40 mins. (Adjust the time by checking the cake during the baking as it will be different depending on the type of the cake tray and the oven.)