Grease the springform pan (Ø 26 cm) well. Preheat the oven to 190 C / 375 f  degrees top/bottom heat.

Combine flour, baking powder, butter, sugar, salt and 1 egg in a bowl and combine with your handheld mixer using the hook attachment. If making the pastry with your hands, use cold butter. Form the pastry to a disk and let chill for 30 minutes or while you prepare the rest.

Clean the rhubarb and cut into small pieces, gather on a microwave safe plate and sprinkle with 2 tablespoons of sugar. Microwave for 30 seconds, mix briefly, then microwave for another 30 seconds. Drain the rhubarb pieces in a colander.

To make German vanilla custard you can simply cook the pudding using cornstarch instead of the pudding powder.

To flavor the custard, you can use vanilla essence but it would be even more delicious to add vanilla bean paste or the seeds of one whole vanilla bean and (depending on the size of your eggs) about 3 egg yolks.

Measure out 400 ml of milk and heat with the seeds of a vanilla bean. Add a pinch of salt. Beat 3 egg yolks with 70 g powdered sugar for at least 3 minutes with a handheld mixer until thick. Stir in 40 g cornstarch.

Add this mixture to the hot milk while stirring constantly and bring to a boil again.

Stir in the sour cream and remove from heat. Fold in rhubarb.

Roll out the dough in a 26 cm springform pan, pulling up the edge slightly. Spread the pudding and rhubarb mixture evenly over the dough.

Bake at 190°C - 375 F for about 45 minutes.

About 10 minutes before the end of the baking time make the meringue.

Add egg whites to clean bowl. For a thick layer or meringue use 4 egg whites. Note that 4 whites might have to bake a bit longer . Add ½ tsp. cream of tartar before you begin beating the egg whites. You can make the meringue without cream of tartar, by adding some salt or lemon juice but adding cream of tartar helps stabilize the meringue and prevent it from weeping.

Add sugar gradually. Add the sugar bit by bit, while beating on high speed. The sugar must be added gradually while you beat the egg whites to stiff peaks. Adding the sugar too quickly will knock the air out of the egg whites and make them difficult to mix thoroughly.

When is my meringue done? When you lift the beaters, the tips will stand straight up. The mixture should be smooth when you rub it between your fingers with no undissolved sugar grains!

When the cake is done take the cake out of the oven and turn the oven down to 160 C  / 300 F. Then run a knife between the cake and the edge of the baking pan.

Remove the edge of the springform pan. Using a spatula, spread the meringue mixture onto the cake and spread decoratively in wavy movements. Finish baking for another 8 minutes depending on how thick you applied your meringue.

Let cool before slicing.