Cake Sponge:

1. Preheat your oven to 350F (conventional) and prepare a 9x13" metal cake pan with parchment paper on the bottom (no butter, oil or spray)
2. Separate the egg whites and yolks into 2 clean bowls, taking care not to get any yolk into the whites
3. To the yolks, add half of the sugar, salt, vanilla, and milk. Whisk for 2-3 minutes until light in color
4. Sift in the flour, cornstarch and baking powder, and gently mix until just combined
5. Add the cream or tartar and remaining half cup of sugar to the egg whites, and use a clean mixer to whisk on medium speed until a glossy, stiff meringue is formed (you should be able to hold the bowl upside down without the meringue falling out). This process can take several minutes, please read my tips in the blog post above to ensure the meringue is perfect
6. Add 1/3 of the meringue to the remaining cake, and use a rubber spatula to very gently fold it in. Then add the rest of the meringue and continue to very slowly and gently fold it in. Take care not to knock out the air or overmix the batter; it should be very light and fluffy
7. Pour the batter into the prepared pan and bake for 15-18 minutes until a toothpick inserted in the middle comes out clean
8. Let the cake cool down in the pan - it is normal for it to deflate a bit; as long as it is still soft and light, it will be fine! 

Soaking:

1. Mix the ingredients of the three milk mixture in a bowl. Taste and adjust as needed
2. Keeping the cake in the pan, poke holes in it using a skewer
3. Slowly pour half of the three milk mixture onto the cake inside the pan. If baked properly, it should soak up the mixture quickly. Wait 10 minutes, then pour over the remaining milk mixture
4. Keep the cake in the fridge for 1-2 hours until it has absorbed all the liquid

Decoration:

1. Whisk the heavy cream to medium peaks and transfer to a piping bag. Pipe swirls on top of the cake (alternatively, just spread it with a spatula). Decorate with berries however you like
2. Slice up & enjoy!