Line the bottom of your cake pan with baking paper.
Heat butter in a small saucepan using low-med heat, stirring gently until it just begins to bubble. Remove from heat.
Add cake flour to butter. Stir quickly to form a paste. Transfer into a large mixing bowl.
Add milk and mix well.
Add vanilla bean paste and salt.
Whisk in egg yolks by hand one at a time, stirring well after each addition to form a shiny and smooth batter. The gelatinised dough will look stretchy and glossy, webbed against the whisk.
Place egg whites in a clean mixing bowl. Beat egg whites until frothy.
Add cream of tartar and continue to beat until foamy. Slowly add sugar in batches and continue beating until firm peaks are formed. Take care not to overwhip or it will be hard to fold in the next step. *Note
Preheat oven to 150ºC and prepare your water bath.
Add one third of the egg white to the yolk batter and mix well. Using a spatula, fold the remaining egg white in two batches. Do so gently and thoroughly so no streaks of egg white remain.
Pour the batter into the cake pan and smooth the top with a spatula. Tap pan against the tabletop to get rid of any large air bubbles.
Put the cake pan into the water bath, ensuring that the water height reaches at least a third of the cake pan.
Bake in preheated oven for 10 minutes then reduce temperature to 140ºC and bake for another 40 minutes or until a skewer inserted in the center of cake comes out clean.
Remove from oven and tap the pan against the tabletop a few times to prevent shrinkage. Transfer to cool on a wire rack pan for 10min before unmoulding.
*If you overwhip, egg whites look grainy and dull. Adding the sugar with the egg whites help to stabilise the egg whites, so here you don’t have to worry as much!