Make a caramel. Pour the sugar on a skellet and let it melt until liquid on medium heat.
Pour your caramel into your baking pan. Use a round 8 inch baking pan for this recipe.
In a separate bowl. Mix all the custard ingredients until smooth.
Pour the custard mixture into the baking pan with caramel. Dont mix!
Make the base for the sponge cake. In a bowl, mix the powdered sugar with the egg yolks until smooth. Then add the oil, water and vanilla. Mix well.
Add the shifted flour and salt. Mix until you dont see any white powder left. Dont overmix.
Make the meringue in a separate bowl. Beat the eggwhites, powdered sugar and cream of tartar until soft peak.
Take 1/3 of the meringue and mix by folding the into the sponge base. Fold the rest 1/3 at a time.
Pour the sponge cake on top of the custard mixture. Don't mix!
Cover the pan with aluminum foil to keep the water from the cake.
Steam for 50 minutes. Check if the sponge cake layer is cooked using the toothpick test. Steam for another 10 minutes if the sponge cake layer is still wet.
Once done steaming, remove the pan from the oven and carefully remove the foil so to not drop some water on to the cake. Immediately run a knife around the side of the cake to release it from the pan.
Use a serving plate that can accommodate the size of your custard cake. Place it on the pan and quickly but carefully flip the pan and plate together. Quickly so not to spill the caramel.
Let it cool for a few minutes at room temperature. Chill before serving.