Mix flour, coconut flour, xylitol and butter in a bowl by hand until the mixture is crumbly. If your butter is not salty, add a pinch of salt.
Place 75g of the mixture in a separate bowl and refrigerate. These are the crumbs for later.
Add vegetable milk to the rest of the dough and mix to a dough. Put the dough in the fridge for about 30 minutes.
Preheat oven to 180 Celsius.
Melt the coconut oil in a pan and grease the tray with it. Remove the dough from the fridge and form a cake crust (directly in the tray). I first formed the edge, pressed it down nicely and then the base. Place the formed crust in the fridge for another 10 minutes.
Put the frozen rhubarb in a pan, cover the bottom of the pan lightly with water and simmer covered for about 10-15 minutes at medium heat until the rhubarb is soft enough.
Pierce the bottom of the cake with a fork and bake the crust on the lowest rack for 10 minutes.
Sieve the cooked rhubarb and collect the cooking liquid. Mix rhubarb pieces with grated coconut, xylitol, vanilla extract and oat flakes. Add 3 to 4 tablespoons rhubarb boiling water if it is too dry. The mixture should be puree-like. If you want a pink/pink colour, add raspberry powder.
Take the cake out of the oven, pour in the rhubarb filling, smooth it down and sprinkle the crumbles over it. If they are too dry, add a tablespoon of rhubarb boiling water so that you can form nice crumbles.
Put the cake back in the oven. Bake for 15 minutes on the lowest rack. Bake for another 10 minutes on the middle rack.