Pre-heat the oven to 160 °C/320 F. Grease a 20cm x 20cm (8 x 8 inch) square baking tin with butter and line with baking paper. The baking paper should extend up the sides of the tin, for easy cake removal.
Place the egg yolks into a large mixing bowl with the caster sugar. Beat with an electric beater for 5 minutes, until the mixture is thick and pale.
Add the vanilla and melted butter and beat on low speed until combined.
Add the sifted flour and gently mix until just combined.
Heat the milk in the microwave or stovetop, until it is warm to touch, but not hot or boiling. Add the milk to the egg yolk mixture and whisk to combine (or continue to beat and add a little at a time). The mixture will be quite runny. This is normal.
Add the egg whites to a separate mixing bowl and beat with the electric mixer (clean beaters) until stiff peaks form. Add the vinegar and continue to beat for an extra minute.
Using a spatula, gently fold the egg whites into the thin cake batter, one third at a time. There will still be some small lumps of egg whites floating on the top of the batter. This is fine.
Pour the cake batter into the prepared cake tin. Bake for around 40-50 minutes (start to check for doneness at 30 minutes) or until the sides of the cake are set, the top is golden brown and there is a very slight 'jiggle' when shaken. The cake should not wobble too much when tested, or it is not quite ready. Cooking times will vary depending on the oven and tin used.
Remove the cake from the oven and allow to sit at room temperature for an hour before covering and transferring to the fridge. Allow the cake to sit in the fridge for 4 hours (or overnight) before slicing, to allow the layers to set.
When ready to serve, slice the cake into pieces and serve with a dusting of icing sugar or ground nutmeg. You can also serve with fresh berries and/or whipped cream. Enjoy!