Mix the margarine and sugar together. Then one after the other add the eggs, the flour with the custard powder and the baking powder and then the milk and stir until smooth. The peeled rhubarb into pieces, the strawberries into slices.
Line a round springform pan with parchment paper, pour the dough into the mold, spread the rhubarb on it and press lightly into the dough. Bake the cake at 170 ° C for about 50 minutes.
Then let the cake cool down a little and prepare the icing according to the instructions on the packet. Place the sliced strawberries on top of the cake and pour the icing on top.