For the rhubarb tartelettes, first preheat the oven to 180 °C top/bottom heat. Grease the tartelette cups.
Peel the rhubarb and cut into pieces about 5 cm long.
Beat the sugar with the butter until fluffy. Add the remaining ingredients for the batter and mix until smooth. Fill the tartelette cups with it. Put the rhubarb inside and sprinkle the flaked almonds on top. Place in the oven for about 25 minutes. Then remove and let cool.
For the elderberry lemon sorbet, bring water, sugar, lemon juice and zest, and vanilla pulp to a boil and simmer for about 5 minutes. Add the elderflowers, place a lid on top and let steep for several hours.
Mix the resulting syrup with the remaining ingredients and freeze for several hours. Stir every half hour.
Serve rhubarb tartelettes with elderberry lemon sorbet.