Fluffy Sponge Cake / Pie

Combine the butter and milk

Add the sifted flour and mix well

Add 5 egg yolks and 1 whole egg, and mix well

Whip the egg whites with lemon juice on high speed, adding 1/3 of the sugar when large bubbles appear

Continue whipping and add the next 1/3 of the sugar when the meringue forms finer bubbles

Add the final 1/3 of the sugar when the meringue shows some streaks while beating

Whip the meringue until stiff/medium peaks form

Fold 1/3 of the meringue into the yolk batter

Pour the batter back into the meringue and fold gently to combine

Pour into an 18x18 cm square pan (lined at the bottom and lightly greased on the sides)

Bake at 150°C for 1 hour 15 to 30 minutes using a water bath (steam bath) method