For the Hamantaschen Dough: Beat the butter with an electric mixer or in a stand mixer fitted with a flat paddle (my preference) on medium-high speed until smooth and creamy. Add the sugar and beat on medium-high speed until well creamed and combined, scraping down bowl as needed. Beat in vanilla then beat in egg until incorporated. Add the flour in 2 or 3 batches and beat on low speed, then increase speed to medium and beat until very well combined and the dough comes away from the sides of the bowl somewhat cleanly. Don’t worry about overworking the dough; make sure it is combined thoroughly and the dough is coming away from the sides of the bowl. Divide into two portions, shape into flat discs and wrap in plastic wrap. Refrigerate at least 2 hours or overnight.

For the Poppy Seed Filling: Place the ground poppy seeds in a small saucepan and stir in the milk, sugar, honey, lemon and orange zest. Cook over low heat, stirring frequently, and bring to a simmer until thickened, about 5 minutes. Stir in raisins, distributing them well within the mixture, and cook for another minute or two. Remove from heat and stir in lemon juice and vanilla extract. Cool slightly, then refrigerate in an airtight container overnight or at least until very firm. It should hold its shape when scooped up with a spoon. This makes about 1 ¾ cups (385 g) of filling, which is enough to fill all of the hamantachen.

Assemble the Hamantaschen: Position oven racks in upper and lower third of oven. Preheat oven to 350°/180°C. Line 3 half-sheet baking pans with parchment paper and set aside.

Remove dough from fridge and allow to stand at least 5 minutes at room temperature. Then work it with your hands, kneading it, until you have a smooth, elastic, but still somewhat chilled ball.

Roll out on a very lightly flour dusted surface to 1/8-inch (3 mm) thick (thin!) If the dough is cracking at all, gather it up and knead it a bit more. This dough is ALL about temperature and rolls like a dream when you hit it right. If you are having problems, chances are it is still too cold. You can also roll out between lightly flour dusted parchment paper, if you find that more helpful.

Cut out rounds with a straight-edge 3-inch (7.5 cm) cookie/biscuit cutter. Brush the edges of the dough rounds with the beaten egg. Place 2-teaspoons of filling in the center of each dough round (I use a tiny little scoop that I also use when making truffles). 

Bring one part of the dough edge up and against the filling, then bring up the other “sides” creating as equilateral a triangle as possible. Firmly pinch together the 3 corners of your Haman’s hat! If the dough is cracking at all at this point, it is because it is too cold.

Bake for about 20 minutes, two pans at a time (don’t try and crowd three racks and trays in at once) rotating the pans halfway through. The cookies should be very lightly tinged with color. Cool completely on pan set on cooling rack. Store cookies in single layers in airtight containers for up to 4 days.