Preheat the oven to 160° C w/ fan.
Grate the zest of an organic lemon & add to the sugar.
Clean & trim the rhubarb; cut the rhubarb in thin slices.
Line the muffin tin w/ paper muffin cups (2 for each muffin).
Mix the the butter, the sugar, the vanilla sugar & the lemon zest until smooth.
Add eggs & mix.
Add flour & baking powder & mix.
Add sour cream & mix.
Fold in the rhubarb.
Fill the dough in the muffin cups.
Bake for about 30 min & make the famous test w/ the wooden pin... Add 5 min if necessary.
When the muffins are at room temperature mix icing sugar & raspberry jam w/ an egg whisk.
OPTIONAL: Add 1 tbsp lemon juice - otherwise if too dry add milk or water.
Scoop the glaze generously over the muffins.
Add decorating sugar if you like.