Mix until it forms a dough.
Form the dough into a couple of spheres and refrigerate until firm, usually an hour or two.
Roll out on parchment paper or a lightly floured surface. I sandwiched the dough between to sheets of parchment. Using a 2.5 or 3-inch cookie cutter, make the circles. At this point, I refrigerated the sheets of cutouts to firm up again.
There are many fillings for these cookies. I used prune lekvar because I happened to see it while shopping at Pomegranate, an upscale Kosher restaurant in Brooklyn.
Place a teaspoon, or less, of filling in the center of each cookie.
There are many ways to create the triangle shape. At first, I used the old-fashioned one, below, of pinching the sides together.
Brush the formed cookies with egg white before baking.
But my first batch was a fail. See how many opened up?
But then I discovered a webpage on how to make them perfect. You can see it here. In the tutorial, Tori Avery demonstrates a folding technique. You can see how much better my second batch looked following those instructions. (There was no third batch, as I ate the dough raw for dinner!)
Production notes: I halved this recipe, using only butter and not Crisco, and followed it exactly save for mixing the sour cream and egg yolks together first. I brushed the tops with egg white before baking. Each batch took about 12 to 15 minutes. (Eleanor Applebaum was the rabbi's wife at Temple Israel, and an editor of this cookbook.)