Add the sugar to the cooking pot. Turn the fire on a medium heat and let the sugar melt into a caramel. You don't have to stir or mix the sugar. Just make sure the fire is not too hot.
In the meantime, you can add the egg yolks, cornstarch and 100ml of the milk to a bowl. Use a whisk to mix everything and to remove the lumps.
Once the caramel is brown you can add 300 ml of milk and the vanilla extract. Let the milk come to a bowl and dissolve the caramel in the milk.
Slowly pour the hot milk into the bowl with egg yolks. Use the whisk to mix the custard, whereafter you can pour everything back into the cooking pot.
Bring the custard to a boil again and let it cook for about 3 more minutes, while whisking continuously. Flavor the custard with coffee powder and sea salt. Pour it through a fine sieve, which is placed on a bowl, and let it cool down in the refrigerator. Place some plastic foil on the custard while it is cooling down.