Let the butter at ambient temperature for 30 minutes, so it gets soft. Then put it with the sugar and the yolks in the stand mixer. (Switch on the oven, at 180 °C, so it will have the right temperature).
Mix them at low-medium speed, until you get a cream.
Chop the hazelnuts in the mixer. Then add them, the flour and the vanilla flavored baking soda.
Mix them at low speed, until you get a very thick dough. Remove it from the standing mixer into a large bowl.
Beat the egg whites in the standing mixer until they are completely firm. Their thickness is crucial for a good final result.
Add the beaten white eggs to the cream in the large bowl.
Gently mix them with a spoon, with a bottom to top movement. Be careful, at the beginning it will be tough, but in a couple of minutes you will get a very creamy mixture.
Anoint the cake pan with a bit of butter, then pour the mixture into it.
Put the pan into the oven, and leave it for about 45 minutes. Use a stick to check if it is cooked.
Leave it to cool, then cut it into two layers. Spread the jam on the lower one, then close the cake back.
Spray the top with icing sugar, then... your first buckwheat cake with cranberry jam is ready to be served!