Peel the rhubarb. Simmer the skins in the sugar liquid for 3 minutes, add lemon juice and set aside to cool. Cut the rhubarb into cubes and put in a vacuum bag with the rhubarb infusion and seal. Steam the rhubarb in a steam oven at 80°C for 3-5 minutes. Remove when ready and leave to cool.
Heat the milk to 37°C, add the yeast. Mix the remaining ingredients in a mixing bowl and blend to a smooth and shiny dough (about 10 minutes). Roll out the dough to 15 mm thickness. Punch out rings from the dough with a pastry cutter (according to instruction). Allow the dough to prove for 45 minutes, then deep fry at 170°C. Season the donuts with fennel sugar.
Heat the milk. Make a caramel of sugar (120g) and water. Add the warm milk to the caramel and simmer the milk until the sugar has dissolved completely. Cook the butter until golden brown and add the milk powder. Mix sugar (50g) and egg yolk in a bowl. Pour the warm milk mixture over the egg and then return to the saucepan and heat the mixture to 85°C under constant stirring. Strain the liquid over the browned butter (Beurre Noisette).
Heat the milk with the seeds from the vanilla pod. Mix egg yolk, sugar and corn starch. Pour the vanilla milk over the egg mixture and whisk. Return the liquid to the same saucepan, heat on medium heat under constant whisking until the first bubble appears. Take off the heat and strain the vanilla cream. Allow to cool in the fridge.
Slice the rhubarb thinly on a mandolin and rinse in cold water. Pick the fennel dill in ice water. Roast the buckwheat in the oven on 140°C until golden brown.