Mix the margarine, sugar, salt, 1 egg yolk, lemon zest and juice together until frothy. Sifted flour and baking powder, add milk, knead the dough lightly. Pour the dough into the mold, prick the bottom lightly with a fork, and put the rhubarb in small cubes on top. Bake the whole thing at 200 ° C for 15 minutes. Then 3 tablespoons. Mix the sugar and the coffee cream and after these 15 minutes pour over the rhubarb, bake for another 20 minutes.
In the meantime, add the egg white to 2 tablespoons. Mix the sugar, beat very stiffly and distribute the egg whites over the cake and bake the whole thing again for about 10 minutes, until the egg whites are light brown. Allow to cool in the mold.