Step 1

Whisk baking powder, salt, and 4 cups flour in a medium bowl. Using an electric mixer on medium-high speed, beat butter and sugar in a large bowl until pale and fluffy, about 5 minutes. Add 2 eggs one at a time, beating to combine after each addition and scraping down sides of bowl.

Step 2

Reduce speed to low and gradually add dry ingredients; mix until dough comes together. Divide dough in half and form into two ¾"-thick disks. Cover and chill at least 2 hours.

Step 3

Place racks in lower and upper thirds of oven; preheat to 350°. Let 1 disk of dough sit at room temperature until softened slightly, about 30 minutes.

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

Lightly beat remaining egg in a small bowl to blend. Working a few at a time, brush edge of rounds with egg, then place 1 1/2 tsp. filling in center. Fold sides up to make a triangle, pinching points gently to seal and leaving about 1" surface of filling exposed.

Step 6

Brush sides of folded dough with egg. Bake cookies, rotating baking sheets halfway through, until bottoms are golden brown, 18–22 minutes. Let cool on baking sheets.

Step 7

Do ahead: Dough can be made 2 days ahead; keep chilled. Cookies can be made 2 days ahead; let cool and store airtight at room temperature.