Using a flour sifter, sift all-purpose flour and cornstarch in a large bowl and place aside. Soak gelatin sheets in cold water for 10 minutes.

Separate the egg yolks from the whites using an egg separator and place the yolks in a mixing bowl. Add sugar and beat it with a hand whisk for 20 seconds. Then add the sifted dry ingredients and whisk until the mixture is thick and whitish.

Pour milk into a heavy-bottomed saucepan. Split a vanilla bean in half lengthwise, scrape the seeds with a knife, and add them and the pod to the milk. Cover the pot with a lid and bring milk to a boil over low heat to infuse the vanilla flavor into the milk.

Discard the split vanilla bean. Remove milk from the heat, and slowly pour 2/3 of the milk over the yolk and sugar mixture to temper the eggs and mix. 

Pro tip: Make sure to pour milk gradually and whisk constantly. Otherwise, the mixture might curdle and resemble sweet scrambled eggs. Just remember that egg yolks start to cook at about 60-70°C.

Pour the tempered yolk mixture back into the remaining milk and mix well. Return the saucepan to low-medium heat and stir vigorously with a hand whisk to prevent the cream from sticking to the bottom of the pan.

Cook the mixture over medium heat for 2 minutes until thickened. The cream is ready when it reaches 179°F (82°C), at a maximum of 185°F (85°C) if checked with a cooking thermometer.

Note: The cream should boil for 2 minutes. While the gelatinization of wheat starch occurs from 52 to 64°C, gelatinization of cornstarch - from 62 to 74°C, the boiling temperature is still necessary for the thickening of the cream. Also, boiling for at least 1.5 minutes is required to sterilize the cream.

Optionally, add the drained gelatine (optionally) to the hot cream and mix to dissolve it. Add very cold butter, cut into small pieces, and mix.

Pass the cream through a fine sieve, pour it into a shallow dish, and cover with cling film on contact. The plastic wrap should touch the surface of the cream to prevent the formation of the crust. Refrigerate for at least 2 hours.

To use, whip the cold pastry cream with an electric mixer for 2 minutes or vigorously with a hand whisk. This will aerate the cream and restore its homogeneity and suppleness before using it.