Very easy, rather good.
c. 500g rhubarb (I used less and it was fine)
500 g polohrubé mouky / plain flour (if you have self raising, use this and omit baking powder!)
250 g sugar, or 200g does nicely for not such a sweet cake.
half a pat of butter or similar, around 100g but it can be approximate
250 ml milk
1 sachet = 2 to 3 tsp baking powder
2 sachets = 5 to 6 tsp vanilla sugar
1 tsp ground cinnamon
Chop the rhubarb into smallish chunks and sprinkle them with the vanilla sugar. Set aside while you make the rest.
Cream butter and sugar, then add egg, flour, baking powder, and as much of the milk as you need to form a fairly soft mixture. Spread this into a greased lined tin, and on top scatter the rhubarb. Sprinkle the cinnamon gently over the top. Bake in a moderate oven, around about 180C, for about half an hour or whatever it takes for the cake to be nicely golden and the cake tester to come out clean.
For those who prefer their rhubarb sweeter, sprinkle with icing sugar after it comes out of the oven.