Mix the butter with 150 g brown sugar and vanilla sugar until frothy. Then add the 2 eggs and the egg yolk. Mix the flour with baking powder and cornstarch and add in portions. Pour the dough into a greased springform pan and smooth it out. Cover with the prepared rhubarb.
Put in the oven preheated to 180 ° C top / bottom heat and bake for almost 1 hour. Let the finished cake cool down a little.
Beat the egg whites with the salt until stiff. Then stir in the lemon juice and stir in 60 g of sugar and powdered sugar. Spread the cleaned strawberries on the cake. Put the egg whites on top and smooth out. Put back in the oven and bake for another 15 minutes to the end.