Day 1:

1. Melt the  butter, mix in sour cream, flour and egg yolks. 
2. Shape into small balls and leave in fridge until the next day.

Day 2:

1. Beat the egg whites until frothy, add the crushed walnuts, icing sugar, vanilla extract and baking powder and mix gently.
2. Roll the small balls until very thin, place about 1 to 2 teaspoons of the mixture on each dough, roll them up and seal the ends.
3. Place them on a baking sheet lined with parchment paper.
4. Bake at 180°C or 350°F until golden brown (20 to 30 minutes).