In the mixer bowl with a dough hook, combine water, yeast, sugar, egg, egg yolks, sour cream, and vanilla. And use a whisk to combine well.
Then, add the flour and salt – combine well with a spatula.
Start the mixer on and knead until everything forms a nice ball.
Then knead for 2 minutes more.
After 2 minutes, start adding the room temperature butter – one cube at a time.
Knead until all the butter is incorporated. Then knead 2 minutes more.
This should now be a beautifully soft, elastic and shiny dough (see video).
Place in an oiled bowl and cover with cling wrap.
Leave the dough in the fridge for 4 to 6 hours (overnight works great).