An Eastern relative of rugelach, these cookies are delicate and complex, filled with sweetened ground nuts and showered with snowy confectioners' sugar. They age well but may never get the chance, given how good they taste.
For the dough: In a large mixing bowl, whisk together the flour and salt. Cut the butter into the flour until the mixture resembles fine crumbs.
Mix in the sour cream, egg yolks, vanilla, and sugar.
Turn the dough out onto a lightly floured surface or piece of parchment and knead lightly to bring it together.
Break off pieces of the dough (a scant tablespoon each) and roll into 3/4" balls. Place the balls on a rimmed, parchment-lined baking sheet, cover, and refrigerate for at least an hour, or as long as overnight.
To make the filling: Place the walnuts in a food processor with the sugar. Process until the walnuts are chopped quite fine. Add the egg whites and vanilla and pulse until smooth.
Preheat the oven to 350°F. Line two baking sheets with parchment.
To assemble: Working with a few balls of dough at a time, flour your hands and press each into a rough oval. Roll 1/8" thick, about 3 1/2" long and 2" wide.
Drop a rounded 1/2 teaspoon of the filling on one of the ends of the oval and spread it halfway up its length. Starting at the covered end, roll each up in a spiral. Transfer to the prepared baking sheets, seam side down.
Bake for 22 to 24 minutes, or just until lightly browned. Remove from the oven and cool on a rack; sprinkle generously with confectioners' sugar while still slightly warm.
