To make the yeast dough, warm the milk to around 35°C/ 95°F. The milk should not be too hot as otherwise the yeast will not rise. Mix the yeast and the sugar into the milk and leave to rest for about 10 minutes. After that you should see little bubbles forming on the surface.
Now add the milk mixture to the flour, egg yolk, salt, nuts and butter. Using a stand or hand mixer with the dough hook attachment, knead the mixture to a smooth yeast dough.
Cover the bowl and leave to rest for at least 30 minutes.
In the meantime, prepare the red currant topping. Mix the butter, confectioners/ icing sugar and vanilla sugar (or extract) until it starts to smooth cream.
Wash the red currants and remove them from their stems. The easiest way is to do that with a fork (see picture)
Prepare a tart or shallow pie dish (27 cm or 10 inch diameter) by greasing it with butter. Sprinkle the form with breadcrumbs (this will prevent the cake sticking later).
With a rolling pin, roll the dough out to the size of the dish. Make sure to remember that you need to cover the edges of the tart form too. About 3-4 mm thick. Place the dough into the cake form and leave to rest for a further 10 minutes.
With your index fingers make little indentations into the dough (see pictures). Fill the little holes with the vanilla , butter mixture. You can fill it with a spoon, or to be more precise you can use a piping bag. Now spread the red currant berries evenly on top of the cake base.
Bake the cake in a preheated oven at 160°C/ 320°F (fan setting) for around 30 – 35 minutes. The exact baking time varies by oven. To check if your cake is done, stick a skewer into the cake. If it comes out clean, it is done.
Leave the cake to cool down a little before serving.