Process the sugars in the food processor until they are finely ground. (Process for about 1 minute. This is how you turn ordinary white sugar into superfine sugar, and it will make the granules of the raw sugar, if you use it, much smaller.) Add the butter to the sugar(s) and process until they are well mixed. Add the flour, salt, and baking powder and pulse the processor until the mixture forms tiny pea-sized crumbs. Add the egg or egg substitute, vanilla, and sour cream mixture to the crumbs and pulse again  – about 10 times – just until everything is combined. Mixture will still be crumbly.

Pour the crumbs into the food storage bag, close the bag (Ziploc works best) and knead the mixture for a short time with your hands. Divide it into 2 equal balls, loosely wrap them in plastic wrap and press into disks. Then freeze the disks for at least 15 minutes or refrigerate the disks for at least 30 minutes. Can refrigerate overnight at this point.

While the dough is chilling, preheat the oven to 350 degrees F. Then make the Nutella filling by putting all the ingredients in the large bowl and mix them together, preferably with a mixer. If you do use a mixer, start slowly so that the flour doesn’t fly out of the bowl. The mixture will be crumbly. 

Form about 24 balls of dough about ¾ – 1 tablespoon of dough per ball and refrigerate them on a large plate or quarter sheet pan for a few minutes while you roll out one disk of the dough. (Form the rest of the Nutella filling into a log and put it in plastic wrap. You can either refrigerate the log, then cut it into coins to bake individual Nutella cookies along with the hamantaschen or freeze it for baking later.)

Remove one disk of dough from the refrigerator. On a lightly floured mat, board, or counter, roll the disk until it is about 12-13 inches in diameter and ⅛ inch thick.

Cut 3 1/2-inch rounds as close together as possible, and place them on the lined cookie sheets, about 6 – 8 per sheet. Gather up dough scraps and re-roll them to use up all the dough. Repeat with the other disk. Chill the dough whenever necessary to keep it cool. Once you have done the first 6-8 hamantaschen dough circles, form the remainder of the disk back into a disk and refrigerate it until ready to make the next batch.

Lightly brush the egg yolk or other wash around the outside of each circle, put a Nutella filling ball in the middle and gently press it down. Pull together two sides of the circle and pull up the third, making a three-corned dough “hat” with the Nutella in the middle.  Pinch each corner to seal it and lightly brush the outside with more egg or other wash. 

If you have time, refrigerate the formed hamantaschen before glazing them with egg wash and baking them. This step is optional, but it does help keep the hamantaschen well formed while baking. When you take them out of the refrigerator, pinch the three corners of the "hat" again to make sure they are well-closed.

Bake one sheet of the hamantaschen at a time, each for about 20-22 minutes on the middle rack of the oven (turning it halfway through), until the outside corners get lightly brown. If you own enough baking sheets, double them up to make a thicker baking surface that helps prevent the bottoms from burning.

As the first batch is baking, take out the remainder of the first disk and make the second batch. Continue the pattern until you finish making all the hamantaschen. You will make 3-4 sheet pans of hamantaschen, with 6-8 hamantaschen on each sheet.