Cream butter, sour cream, and sugar. Add egg yolks, then flour. Knead with hands until well blended, adding a little more flour if needed. Divide dough into 6-8 sections, pat each into a flat piece, then chill. Roll out, between waxed paper, each section like a pie crust. Cut with a sharp knife 4 cross cuts to make 8 triangles. Drop 1/2 teaspoon of prune butter onto each triangle, roll up starting at wide end. Put on greased cookie sheet.