For the Cookie Dough

1. In medium bowl, whisk together the flour, baking powder and salt.
2. In the bowl of an electric mixer, beat the butter and sugar until light and fluffy, about 3 minutes. Scrape down the sides of the bowl with a rubber spatula, then beat in the egg, egg yolk, and vanilla. On low speed, beat in the flour mixture until just incorporated, scraping down the sides of the bowl as necessary. Scrape the dough onto a very lightly floured surface (just a few pinches of flour) and knead with your hands a few times until smooth. Divide the dough into 2 pieces and form each into a 6-inch disc. Wrap each disc in plastic wrap and refrigerate until firm enough to roll, a few hours or overnight.

For the Chocolate Filling

1. Place the butter in a medium microwave-safe bowl. Microwave on high until melted, about one minute. Add the chopped chocolate and stir until the mixture is completely melted and smooth (zap again in the microwave for 20 seconds if necessary). Stir in the sugar, vanilla and salt. The mixture will look grainy -- that's okay. Add the egg and stir until incorporated. Stir in the flour and beat with a spoon until the mixture is smooth, about 30 seconds. Cover and refrigerate until ready to use.

To Assemble

1. Preheat oven to 350°F and position racks in upper and lower thirds of the oven. Line two baking sheets with parchment paper.
2. Remove 1 disc of dough from the refrigerator and let sit at room temperature until supple enough to roll, about 15 minutes. Place the dough on a very lightly floured work surface and knead with your hands a few times to soften it up. It will continue to soften as you work with it. Roll to about ⅛-inch thick, turning and dusting with more pinches of flour as necessary. (Take care not to use too much flour, and don't worry if the dough starts to crack at the edges; just cut the cookies from the inside area. When you re-roll the scraps, the dough will be softer and less prone to cracking.) Using a cookie cutter or glass, cut out 3-inch circles and place them about ½-inch apart on the prepared pans. Re-roll and re-cut any excess dough. Scoop and place a level teaspoon of the chocolate filling in the center of each cookie (do not overfill). Bring 3 sides of each cookie up to partially cover the filling, then firmly pinch the edges of the cookie to seal the corners and form a triangle. Place the unbaked hamantaschen in the freezer for 10-15 minutes to firm up (if your pans don't fit in your freezer, you can place them in the fridge for 30 minutes).
3. Bake for 12 minutes, turning the pans halfway through baking, until lightly golden around the edges. Let cool on the pan for a few minutes, then transfer to a rack to cool completely. Repeat with the remaining dough until all of the cookies are baked.
4. Note: It is very important to use the spoon and level method for measuring the flour for this recipe, or your dough will be dry. Use a large spoon to lightly spoon the flour into the measuring cup until it is slightly overflowing, then use the flat side of a knife to level it off so that it's even with the top of the measuring cup. Do not tap the cup, as this causes the flour to settle in and become more dense.
5. Note: The cookies are best on the day they are made but will keep for a few days in an airtight container. Cool the cookies completely before stacking or storing.
6. Freezer-Friendly Instructions: The Dough can be Frozen for up to 3 Months: Shape the dough into 2 discs, wrap each securely in plastic wrap, and place them in a sealable bag. When ready to bake, remove the disks from the freezer, thaw the dough for 1 to 2 days in the fridge, and then proceed with recipe. To Freeze After Baking: Let the hamantaschen cool completely and store in an airtight container separating layers with parchment paper or aluminum foil. Before serving, remove the cookies from the container and let them come to room temperature.