For filling: In a spice grinder, course of poppy seeds, 2 tablespoons (24 grams) granulated sugar, and salt till finely floor, 3 to 4 minutes.
In a small saucepan, warmth milk and butter over medium warmth till bubbles kind round sides of pan. (Don’t boil.)
In a medium bowl, whisk collectively egg yolk, lemon zest, and remaining 2 tablespoons (24 grams) granulated sugar. Slowly add scorching milk combination, whisking consistently. Return combination to pan; prepare dinner over medium-low warmth, whisking often, till thickened, 1 to 2 minutes. Pressure combination via a fine-mesh sieve right into a medium bowl. Stir in poppy seed combination. Cowl with a bit of plastic wrap, urgent wrap immediately onto floor of filling to forestall a pores and skin from forming. Refrigerate till chilled, no less than 1 hour.
For dough: Within the bowl of a stand mixer fitted with the paddle attachment, beat butter, confectioners’ sugar, and lemon zest at medium pace till creamy, 3 to 4 minutes, stopping to scrape sides of bowl. Beat in egg and vanilla.
In a medium bowl, whisk collectively flour and salt. With mixer on low pace, steadily add flour combination to butter combination, beating till a clean dough types. End up dough, and press right into a rectangle (about 1 inch thick). Wrap in plastic wrap, and refrigerate till agency, about half-hour.
Line 2 rimmed baking sheets with parchment paper.
On a evenly floured floor, roll dough to ¼-inch thickness. (If the dough is simply too stiff, let stand at room temperature for quarter-hour.) Utilizing a 3¼-inch spherical cutter, minimize dough, rerolling scraps as crucial. Spoon 2 teaspoons (10 grams) filling into heart of every dough circle. Fold dough in from three sides, overlapping at corners, and pinch edges collectively to seal, leaving a small opening over filling. Place 2 inches aside on ready pans. Refrigerate for 1 hour.
Preheat oven to 350°F (180°C).
Re-pinch corners of hamantaschen to make sure they’re properly sealed.
Bake till bottoms and corners are golden, 20 to 25 minutes, rotating pans midway via baking. Let cool utterly on pans.
