Sift the flour into a large bowl, add 1 tsp sugar, the baking soda, baking powder and salt. Mix well, add nutmeg, then stir again. Mash the banana well and mix with the yogurt. Flavor it with vanilla.
Beat 1/2 cup of sugar with the butter, add eggs. Beat the mixture until fluffy and gradually pour in the flour. Once the flour is in the egg mixture, add the banana and mix.
The core should remain sticky. Let the dough rest covered in the refrigerator. Once you take it out, divide it into equal parts, flour them generously, then spread them out in a thin sheet.
With a metal disc or a glass, cut out circles. Fry them in preheated oil. Take the donuts out onto a napkin to absorb the excess oil.