Mix butter and flour until the mixture forms crumbs. (To make even crumbs, many housewives in Hungary slice the butter with a sharp knife or in a vegetable sheer.) Add egg yolk, salt and sour cream. Knead the mixture to make a dough. Wrap dough in wax paper and put in the refrigerator for a minimum of 1 hour.
Remove dough from refrigerator and roll and stretch