Prepare a dough from butter, sugar, flour, egg yolk and baking powder and put it on a greased tray dusted with breadcrumbs. I usually take a baking dish of 20 x 30 cm, then the cake will be a little thicker than on a tray. Beat the egg whites with the sugar until stiff. Cut the rhubarb pieces into strips about 1 cm wide.
Peel and chop the ginger. Fold the rhubarb and the ginger into the egg whites.
Spread the mixture on the dough and then bake for about 45 minutes at 180 degrees convection.
Tastes best warm to me.