Heat 20 cl of milk with vanilla and butter.
In a bowl, dilute the starch with 5 cl of milk, mix well, add the yolks and sugar and mix again.
Then gradually pour the hot milk, mixing well with a whisk.
Put on low heat, stirring constantly, so that it does not stick to the bottom, until the cream thickens.
Pour the cream into a tray and place a cling film on the surface.
Allow to cool and then refrigerate for at least 2 hours.
In a bowl, mix the flour and dry yeast.
Add the preheated milk and knead well (if you have a robot use it).
Add sugar, salt and egg and knead for 10 minutes, until the dough comes off the bowl. Add the butter in 2 stages and knead until well incorporated.
Cover the bowl with a towel and let it rise for 60 minutes in a warm place.
Flour the work surface, knead the dough a little.
Roll out the dough with a rolling pin, cut the donuts with a mold or a glass.
Heat the oil and fry the donuts for 30 seconds on each side.
Remove them on absorbent paper.
You can consume them simply powdered with powdered sugar or you can fill them with vanilla cream.
If you fill them, keep them cold and powder them with sugar before serving.