Whip the butter, softened at room temperature or in the microwave, with the sugar.
Combine, one at a time, 6 egg yolks (keep the egg whites apart).
Mix the buckwheat flour with the finely ground almonds and half a packet of yeast.
Beat the egg whites until stiff and gently incorporate them into the prepared mixture.
Pour into a pan of about 28 cm in diameter and place in a preheated oven at 175-180 ° for about 35-40 minutes.
Remove the cake from the oven, let it cool, cut it in half horizontally, spread the base with blueberry jam or berries and put it back together.
Sprinkle with plenty of icing sugar.