Ingredients
Method
Whisk together 3 cups (375 g) all-purpose flour, 1 tsp. baking powder, and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt in a medium bowl. Whisk together 1 large egg, 1 large egg yolk, 2 tsp. vanilla extract, and ¼ tsp. almond extract in a second medium bowl.
Beat ¾ cup (1½ sticks) unsalted butter, room temperature, ½ cup (100 g) granulated sugar, and ½ cup (55 g) powdered sugar in the bowl of a stand mixer fitted with the paddle attachment on medium speed, scraping down sides of bowl, until light and fluffy, about 2 minutes. Reduce speed to medium-low and gradually stream in egg mixture; beat until incorporated, about 1 minute. Scrape down sides of bowl. Increase mixer speed to medium and continue to beat until mixture is thick and smooth (it might look curdled at first), about 1 minute more. Scrape down slides of bowl, then reduce mixer speed to low. Add half of dry ingredients and beat until just incorporated with some floury streaks remaining. Add remaining dry ingredients and beat until incorporated (no floury bits) and dough is homogeneous.
Turn dough out onto a surface and form into a ½"-thick disk. Wrap in plastic and chill at least 1 hour. Do ahead: Dough can be made 12 hours ahead. Keep chilled.
While the dough is chilling, stir 1 Tbsp. finely grated lemon zest and 2 Tbsp. fresh lemon juice in a small bowl to combine.
Bring ½ cup whole milk, ¼ cup (50 g) granulated sugar, and ¼ cup poppy seeds to a simmer in a small saucepan over medium heat. Reduce heat to low and cook, whisking occasionally and scraping bottom of pan, until mixture is thickened and reduced by about two thirds, 10–12 minutes. Whisk in lemon zest mixture and cook, whisking occasionally, until poppy seeds have absorbed most of the liquid, about 2 minutes. Scrape poppy seed filling into a clean medium bowl and chill at least 30 minutes.
Preheat oven to 350°. Unwrap dough and roll out on a lightly floured surface to ⅛" thick. (If dough is very firm, let sit at room temperature a few minutes before rolling.) Punch out rounds with cookie cutter and transfer to 2 parchment-lined baking sheets, spacing at least 2" apart. Gather scraps, reroll, and punch out more rounds; transfer to baking sheets. (You should have about 24 rounds total.)
Place 1 tsp. poppy seed filling in the center of 12 rounds; place 1 tsp. jam (from ¼ cup apricot, prune, or raspberry jam) in the center of remaining rounds. Lightly brush edges of dough with 1 large egg white, lightly beaten. Working with 1 cookie at a time, fold in left and right sides of dough so they partially cover filling and overlap at bottom corner. Fold in top side of dough so it partially covers filling and overlaps left and right sides of dough at corners, forming a triangle. Pinch corners and overlapping edges to seal. Freeze cookies 10 minutes.
Bake cookies, rotating baking sheets top to bottom and front to back halfway through, until golden brown, 15–20 minutes. Let cool on baking sheets. Do ahead: Filling can be made 1 week ahead. Transfer to an airtight container and keep chilled.