In a bowl suitable for cooking over water bath whisk eggs, cornstarch and sugar and put aside.
Meanwhile, heat half a liter of milk with 3 – 4 teaspoons of ground coffee. The amount of teaspoons depends on your personal taste and the type of coffee you are using. Add less coffee in case you want your coffee custard with a less bold flavor.
As soon as the milk is about to reach the boiling point, turn off the heat. Pass it through a fine sieve into the egg mixture to remove almost all the ground coffee powder (don’t worry if it some of it passes through the sieve). Stir and place the bowl over a container full of boiling water to cook the custard over the water bath.
I remind you that the container with the egg mixture must not touch the surface of the boiling water. At the beginning stir often to ensure that there are no lumbs and towards the end of cooking stir continuously using the whisk. Cook over the water bath until the cream becomes thick and shiny. It takes some time so be patient.
If you are having some hard times to get your cream thick that’s because it doesn’t get enough temperature. That happens when the bowl that you are using is too small and there is not enough heat in the bottom or because it is too wide and the cream surface is too big to get hot evenly. If you have this problem just place a lid over the bowl with the mixture for 15 – 30 seconds for a couple of times. That will rise the temperature of the cream and it will get thick but be careful this method can cause lumps so remember to whisk every time.
This type of cream should be cool down quickly to make sure no lumps appear. Prepare a container full of lukewarm water (to avoid thermal shock) which then you will replace by very cold water. To make the job easier, I simply used the kitchen sink filled with water.
Move the bowl in lukewarm water and let it rest for a couple of minutes. Mix and only now add the vanilla extract. Change the cooling water into very cold water and let your custard rest until it is completely cold. Meanwhile, remember to cover the surface of the cream with the plastic wrap to avoid creating the dense part.
This coffee custard should be used immediately, otherwise it should be stored in the refrigerator and used during the same day.