Line the base of a 20-21cm round cake pan with greaseproof paper without greasing the sides of the pan.
Combine UHT low-fat milk, essence and melted butter in a small bowl and set aside (to make a buttermilk mixture).
Whisk the egg whites until it turns foamy, then gradually add in the sugar. Continue to beat the mixture until the egg whites turn thick and glossy. Add in the egg yolks and continue to beat until well blended.
Gently fold in the sifted flour and salt in batches using a metal spoon, stopping once the flour is well incorporated.
Drizzle in the buttermilk mixture and fold in with a metal spoon.
Pour the cake mixture into the prepared pan and level the surface with a spatula. Bake in preheated oven at 180°C for 25-30 minutes or until cooked through.
The cake is cooked when it has shrunk slightly away from the sides of the pan and the surface is golden brown.
Leave the cake to cool in the pan for a few minutes before turning out onto a wire rack.