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.
Bake 375 degrees or till light brown. If desired, sprinkle with powdered sugar. Even though these have very little sugar, they are fabulous. They even give Kolaches some stiff competition.