First of all, prepare the cream: beat the eggs with the sugar, then add the flour first and then the milk and vanilla.
Transfer the mixture to a saucepan and bring to a boil, stirring constantly to avoid lumps, cooking until a thick cream is obtained, then transfer to a bowl, cover with cling film and let it cool.
Now move on to the preparation of the sponge cake: whip all the ingredients together, with the whips at maximum speed, for at least 10-15 minutes: the mixture must be clear, fluffy and at least tripled in volume.
Transfer the mixture into a pastry bag with a medium-large smooth spout and then distribute it in the cups, trying to create a slightly pointed shape in the center.
Bake for 15-20 minutes in a static oven preheated to 180 ° C: if it seems to you that the cakes are coloring too quickly, lower the temperature to 150 ° C.
Let them cool completely, then fill them with the custard creating a hole in the bottom with a sac-à-poche with a long and fairly thin spout.
The nuns' boobs are ready, decorated with powdered sugar and served.