Preheat the oven to 300F (150C). Line with parchment paper the bottom and sides of an 8-inch (20cm) square pan. (preferably not a pan with a removable base, in this case, you would need to wrap the pan with heavy-duty aluminum foil).
Separate whites from yolks and let them get to room temperature.
In a medium bowl, sift the flour and set aside.
In a medium saucepan add butter (or oil if preferred) and milk.
Place over low heat just until butter is melted. Don’t boil or simmer.
Pour the milk mixture over the flour and mix to combine.
Add egg yolks and mix to incorporate.
Add vanilla extract, mix, and set aside.
Add salt over egg whites, and whip until foamy. Gradually add sugar and continue mixing until soft peaks form.
Make sure you don’t over whip, otherwise your cake might get cracked.
Gently fold in the egg yolk mixture.
Pour the batter into the prepared pan and rap the pan a few times to get rid of large air pockets.
Place the pan into a larger baking tray and add warm to hot water, not boiling, into the pan, up to halfway of the pan.
Place into the oven on the second bottom rack, not middle and bake for 1 hour and remove the pan from the baking tray.
Serve cold or slightly warm and enjoy!