Preheat oven to 180C°/160°C fan forced. Grease a 16 x 26cm slice pan (base measurement). Tear a sheet of baking paper about 13cm longer than the pan. Press the paper into the pan, getting neatly into the corners, and smooth out any folds of paper. It should overhang the pan on all four sides.

Reserve 1 tablespoon caster sugar. Place the egg yolks, vanilla and remaining sugar in a large bowl. Use electric beaters to beat for 2-3 minutes or until very pale and creamy. Pour in the butter and beat to combine. Sift in the flours and custard powder. With the beaters on low, beat in the flour until just combined.

Add the milk, a little at a time at first, and use a balloon whisk to mix until evenly combined (it will be a very liquid mixture).

Use clean electric beaters to beat the egg whites in a bowl until soft peaks form. Add the reserved caster sugar and beat until firm peaks form. Use a balloon whisk to fold the egg white mixture into the milk mixture until just combined. Gently pour into prepared pan. The pan will be very full, coming right to the brim, but the paper should help contain it. Carefully transfer to the oven.

Bake for 35 minutes or until golden brown and mostly set, but still a bit wobbly in the middle. Set aside in the pan to cool completely.

Use the long edges of paper to lift the slice from the pan. Cut into squares and dust with icing sugar to serve.