1) In the bowl of a stand mixer, beat the butter, sugar, salt, and vanilla paste until creamed. Add egg yolks and combine well.
2) Combine the baking and plain flour. Beat in half the flour and half the milk then add remaining milk and flour and combine well.
3) Transfer batter to baking tray lined with greaseproof paper or 30-cm round cake tin. Bake about 45 minutes to 1 hour, or until a toothpick inserted in the centre of the cake comes out clean. Assemble cake using the Swiss buttercream.
For the Swiss buttercream:
1) Whisk whites and sugar over a double boiler until mixture reaches 60C.
2) Transfer egg mixture to mixer with whisk and whip on high until room temperature. Add vanilla and butter and whip until fluffy.