Sift together flour, baking powder, sugar, and salt.
Mix in butter and eggs.
Lightly flour work surface and roll out dough to 1/8 inch thick. (If you prefer a cakier hamantashen, roll to 1/4".)
Use a cookie cutter or the rim of a glass to cut hamantaschen dough into circles. You can make them any size that you like. I like to use a 3 1/2" cookie cutter.
Add about 1 teaspoon of filling to the center of each piece of dough for a 3 1/2" cookie. Use more or less if you make a different size.
Use your finger or a pastry brush to moisten the edge of each circle with a tiny bit of water. This will help the cookies stay closed during baking.
Fold down one third of the circle, covering a portion of the filling. Then, fold the next third down, overlapping the first third. Finally, fold down the last third to create your triangle. Press the overlapping areas gently to seal in the goodness.
Transfer to a try lined with parchment paper. Put the tray in the freezer for about 15 minutes. This will help the cookies to hold their shape.
Remove from the freezer and bake for 12-15 minutes or until cookies just start to brown.
Notes
If you like a shinier look to your cookies, brush the top of the triangle with egg before baking.
Always fold to shape the cookies, never pinch.
Have fun experimenting with different fillings. You can use many different fillings in the same batch of cookies. Thicker fillings work better than thin runny fillings. If you want to use a thin filling, use just a little of it to keep it from overflowing out of the cookie.
Store the cookies in a sealed container for up to one week or package them to give to friends in Purim baskets (mishloach manot).