Step 1 Whisk 4 cups (500g) all-purpose flour, and 1 ½ tsp. baking powder in small bowl until well combined.

Step 2 Beat 1 cup (2 sticks) unsalted butter, 1 cup (200g) sugar, 1 tsp. Diamond Crystal kosher salt, and 1 tsp. (packed) finely grated lemon zest in a stand mixer fitted with paddle attachment on medium speed until pale and creamy, about 3 minutes. Add 2 large eggs, one at a time, beating for 30 seconds after each addition and scraping down sides of bowl. Reduce speed to low and add dry ingredients; beat until just combined. Divide dough in half and pat into disks.

Step 3 Working one disk at a time, roll out dough between 2 sheets of parchment paper to ¼" thick. Stack sheets of dough, still sandwiched between parchment, and slide onto a baking sheet. Chill until firm, about 1 hour.

Step 4 Roll out dough on a very lightly floured surface to about ¼" thick, dusting with flour as needed (use as little flour as possible). Cut out 3 1/2" rounds with cutter and, using an offset spatula or bench scraper, transfer to 2 parchment-lined baking sheets. Gather up scraps, reroll, and cut out additional rounds.

Step 5 Place racks in upper and lower thirds of oven; preheat to 350°. Unstack dough sheets and transfer to a surface; carefully peel away top sheet of parchment paper from each. Punch out cookies as close together as possible with cutter. Transfer cutouts to 2 parchment-lined baking sheets (reuse the top sheets from rolling out cookies). Gather up scraps, reroll, and cut out additional rounds, chilling dough briefly in between if needed.

Step 6 Place 2 tsp. of your chosen filling in center of each cookie round. Fold three sides up and over filling (leaving about 1” of filling exposed) to make a triangle, pressing points gently but firmly to seal.

Step 7 Bake cookies, rotating baking sheets top to bottom and front to back, until bottoms are golden brown, 18-22 minutes. Let cool 5 minutes on baking sheets, then transfer to wire rack and let cool completely.