Preheat the oven to 160° w/ fan.
Line the baking tin w/ baking parchment.
Mix sugar, vanilla sugar & salt.
Mix flour & baking powder.
Grate the lemon zest & squeeze the lemon; set aside the lemon juice.
Trim the rhubarb & chop in thin slices.
With the handheld mixer whisk the butter & the sugar.
Add eggs & whisk.
Add lemon zest & whisk.
Add oil & whisk.
Add the mixture to the flour & whisk.
Add sour cream.
Finally fold in the chopped rhubarb.
Pour the mess in the baking tin & sprinkle sugar (1 tbsp) & lemon juice (1 tbsp) on top.
Bake in the oven for about 55-60 min.
Let it cool & remove carefully from the tin.
Whisk together the icing sugar & the lemon juice.
When the cake is at room temperature pour the icing over the top.