Heat the water with the butter, vanilla sugars and a pinch of salt.
When the butter has melted, turn off the heat and add the flour and mix well. Leave to cool for 10-15 minutes.
When it has cooled to lukewarm, add the eggs and sourdough, mix until homogeneous and pour into a meringue bag. I used a decorative tube: Szőregi 887
Meanwhile, start heating the oil, it should be about 170 degrees for frying.
I cut a piece of baking paper into strips and press the strips out of the foam bag onto the paper, then place the strips in the oil. I use tongs to pull the baking paper out from underneath and fry both sides until golden brown. Place the churros on a paper towel to drain off excess oil, then roll in cinnamon sugar while still hot.
Serve with chocolate sauce or whipped cream, whichever you prefer.