1. Preheat oven to 350F
2. Mix the butter into the flour with your hands.
3. Beat the egg yolks with a fork and mix into the butter/flour mixture.
4. Add the sour cream and mix altogether with your hands.
5. Knead until smooth, about 5 minutes.
6. Shape into a log. Cover with food safe plastic wrap and put in the fridge overnight.

For the filling

1. Beat the egg whites until frothy. Add the icing sugar and continue beating until well mixed. Fold in the walnuts and rum if using.

Assembly

1. Divide the dough into 7 equal pieces (keep unused dough covered in the fridge).
2. Roll each piece into an 18X20 sheet.
3. Divide the filling into seven equal parts and spread 1 part of filling onto each sheet.
4. Fold one third of the dough sheet into the middle and then fold the other side over (see photo).
5. Each cookie is 3/4 wide. Make 2X1/4″ slits, being careful not to cut through the ends of the cookie and make the final cut all the way through to form one cookie. See video.
6. Carefully twist the cookies to shape as in the photo.
7. Bake on the middle rack of oven until golden brown in colour. Flip pans, halfway through.