Make the dough by combining the flour, cocoa powder, baking powder, and salt in a large mixing bowl and stir to blend well.
In the bowl of a stand mixer fitted with a paddle attachment, combine the butter and sugar and mix at medium speed for 3-5 minutes until light and fluffy. Scrape down side of bowl with rubber spatula.
With the mixer running at low speed, gradually add the flour mixture, scraping down the bowl as necessary. Add the eggs, one at a time, mixing after each addition, until fully incorporated. Add the coffee, vanilla extract and chocolate liqueur and mix to form a dough.
Shape the dough into 2 flat, round disks, wrap tightly in plastic and refrigerate for at least 4 hours, until very firm. (This dough can be refrigerated up to 3 days before rolling and baking.)
When you are ready to bake, preheat the oven to 350F. Line 2 rimmed baking sheets with parchment paper or nonstick mats and set aside. Lightly four a work surface and rolling pin
Place 1 disk of dough on the work surface and roll out until it is between 1/4 and 1/8 inch thick. (The second dish should remain refrigerated while you do the first batch.) With a 3 inch cutter, cut out as many rounds as possible and place them on the prepared baking sheets, arranging them about 1 inch apart. Collect the scraps of dough, re-roll once, cut the additional circles and place on the baking sheet.
Make the egg wash by whisking the egg whites and 1 teaspoon of water in a bowl. With a pastry brush, brush the edges of each round of dough with the egg wash.
Place the Nutella and crumbled halvah in a small mixing bowl and store until fully combined. It will not be smooth and may have a few small chunks.
Place 1 heaping teaspoon of the filling in the center of each circle. Fold up the edges to shape each filled round into a triangle, pinching them to form points, and leaving a small amount of the filling exposed at the top. Repeat with the remaining chilled dough and filling.
Chill the filled hamantaschen in the refrigerator for 10 minutes.
Bake for 13-14 minutes, until firm, alternating the baking sheets between the oven racks and turning the trays180 degrees halfway through the baking time. Let cool for 5-7 minutes and transfer to a cooling rack to cool completely. Bake the remaining hamantaschen.