In a large bowll, add the butter and flour. Mix with a fork until crumbly.
In a small bowl, combine sour cream and egg yolks.
Add the sour cream/egg yolks to the butter/flour mixture. Mixture until well incorporated.
Shape the dough into 6 even balls and place onto plate lined with parchment paper. Light dust the tops of the dough with flour, cover with plastic wrap and refrigerate at least 2 hours (or overnight).
Cover a baking sheet with parchment paper. Set aside.
In a bowl beat the egg whites (room temp) until soft peaks form, about 2-3 minutes. Slowly add sugar and beat another 2-3 minutes until stiff peaks form.
Chop walnuts, not too small and not too large.
Once egg whites are beaten with a wooden spoon or spatula, gently add in the chopped walnuts to the egg whites. Set aside.
Take one ball of dough (leave the rest refrigerated). On a lightly floured surface roll it out to about 10" inches in diameter.
With a pizza cutter cut into 8 triangular parts. Add the meringue filling to the edges of each triangle.
Gently roll up the rogaliki and place edge end down onto the parchment paper. If adding dried fruit add a little bit of the meringue filling and dried fruit as well. This way they are not dry.
Bake in preheated oven at 365°F about 20-25 minutes, until golden in color.
Add the powdered sugar to bowl. Once rogaliki are baked let them cool just a tad bit and while still warm completely cover with powdered sugar.