Instructions

for the filling

Working in 3 batches, grind poppy seeds in spice grinder until fine and powdery (but stop before paste forms), 15 to 30 seconds per batch (scrape bottom of spice grinder clean between batches). Bring cream, honey, and salt to simmer in small saucepan over medium heat. Stir in ground poppy seeds and cook, stirring occasionally, until mixture is thickened, liquid is absorbed, and seeds begin to clump together, 2 to 3 minutes. Off heat, stir in butter, lemon zest and juice, and vanilla. Transfer filling to bowl and let cool to room temperature, about 1 hour. (Cooled filling can be refrigerated for up to 3 days.)

for the dough

Using stand mixer fitted with paddle, beat butter, sugar, cream cheese, and salt on medium speed until pale and fluffy, about 3 minutes, scraping down bowl as needed. Add egg and vanilla and continue to beat until well incorporated, about 1 minute longer. Reduce speed to low, add flour, and mix until dough is uniform, about 1 minute. Divide dough in half. Transfer each half to sheet of plastic wrap, form into 6-inch disk, wrap tightly, and refrigerate until firm, at least 2 hours or up to 3 days.
Adjust oven rack to middle position and heat oven to 350 degrees. Line rimmed baking sheet with parchment paper. Unwrap 1 dough disk, place in center of large piece of parchment, and cover with second large piece of parchment. Roll dough into 12-inch round of uniform ⅛-inch thickness, occasionally flipping dough and gently peeling off and replacing parchment to avoid creases. Refrigerate rolled dough sheet between parchment while repeating rolling (between parchment) with remaining dough disk.
Peel off top piece of parchment from 1 rolled dough sheet. Using 3-inch round cookie cutter, cut dough into 11 or 12 rounds and space evenly on prepared sheet. Dollop 1 heaping teaspoon filling in center of each round. Working with 1 round at a time, fold 3 sides of dough over filling to create triangle shape, overlapping dough at corners and leaving small window of filling exposed. Gently press corners to seal and press any cracked edges back together. (If dough becomes too fragile to work with, return briefly to refrigerator.)
Bake cookies until evenly golden brown around edges, 16 to 20 minutes. Let cookies cool on sheet for 5 minutes, then transfer to wire rack. Repeat process with second sheet of rolled dough.
Gather dough scraps into rough disk shape and reroll between parchment to ⅛-inch thickness. Repeat cutting, filling, shaping, and baking with rerolled dough; rerolled dough should yield 4 to 8 additional cookies. (Do not reroll scraps a second time.) Let cookies cool completely before serving. (Cooled cookies can be stored in airtight container for up to 3 days.)