Prepare the Dough. In a bowl, whisk together the flour, baking powder, and salt. Set aside.
In the bowl of a mixer, press the lemon zest into the sugar with your fingertips to release the oils from the zest. Add the butter and cream for 2-3 minutes. Add the egg, the egg yolk, lemon juice, milk, and vanilla and beat until all the liquid ingredients are fully incorporated.
With the mixer off, pour the dry ingredients into the mixer bowl over the liquid ingredients. Turn the mixer on the lowest speed and mix just until the dry ingredients are fully incorporated. (Alternatively, you can mix the dry ingredients in by hand using a rubber spatula.) Do not overmix.
Place half the dough on top of a large piece of plastic wrap. Using the plastic wrap, shape the dough into a disk, pushing down on it to thin it out. Fully cover the disk of dough in the plastic wrap and place in the refrigerator for at least two hours and for as long as 24 hours. (I usually make the dough the day before I am going to bake, so I let it sit overnight in the refrigerator. If you are in a rush, you can put the dough in the freezer for 30 minutes instead of the refrigerator.) Repeat with the remaining portion of the dough. While the dough rests in the refrigerator, make the blueberry filling and lemon crumbs.
Make the Blueberry Filling. Pour the blueberries into a small or medium size saucepan. Pour the lemon juice over. In a separate bowl, combine the sugar, cornstarch (or Clear Jel) and salt. Pour over the blueberries. Over medium heat, stir the mixture and cook until the juices on the bottom start to simmer. Reduce the heat to medium-low and continue stirring until the mixture thickens. Remove from heat, pour the blueberry mixture into a heatproof bowl, and allow to cool to room temperature before using. (You can make the filling in advance and work with it directly from the refrigerator. Just give it a good stir before using it.)
Make the Lemon Crumb Topping. In a small bowl, mix the flour, sugar and salt. Add the zest and rub it into the sugar/flour/salt with your fingertips to release the oils in the zest. Add the lemon juice and melted butter. Stir to form crumbs. It should be the consistency of sand with some larger clumps. Refrigerate until ready to use.
Shape Hamentaschen. Line 2 or 3 cookie sheets with parchment paper. Take one of the dough disks from the refrigerator and roll out in between two pieces of parchment paper that are lightly dusted in flour. Roll the dough so that it is approximately ¼ of an inch thick. If you roll it too thin, the dough will rip as you remove it from the parchment. Using a round cookie cutter (I use a 2½ inch diameter), cut as many rounds out as you can. You may need to dip your cutter into some flour to avoid it sticking to the dough. Use a metal spatula dipped in flour to release the rounds. Gather the scraps and roll that clump of dough out and cut more rounds. Keep repeating until you have used all the dough. If at any point the dough becomes too sticky or difficult to work with, cover it in plastic wrap and let it chill in the refrigerator for about 10 minutes. Try to avoid adding too much flour to the dough.
Fill each round with approximately 1 teaspoon of the blueberry filling. Lightly flour your fingertips and form each circle into triangles by folding up the rounds and pinching them tightly in three corners. (If they are not sticking together, wet your fingertip and rub it on the inside where you are trying to create the point.) You want to make sure that the seams are fully closed so that they do not open while baking. Place the shaped hamantaschen on the prepared cookie sheets.
Place the cookie sheets with the shaped hamantaschen in the freezer for at least 15 minutes. DO NOT SKIP THIS STEP. (If you have limited freezer space, place them tightly, right next to each other, on one prepared cookie sheet or a plate. Once frozen, you can easily remove them and set them further apart on another prepared baking sheet for baking.)
Preheat oven to 400° F.
Repeat steps 7 through 9 with the remaining disk of dough.
Top each hamantaschen with approximately 1 teaspoon of the lemon crumb topping. Bake the frozen hamantaschen for 10-12 minutes until they are lightly brown around the bottom edges.
Cool on pan for a few minutes and then completely on a wire rack.