Step 1

Finely grind poppy seeds in spice mill. Transfer to a medium saucepan and add honey, milk, lemon juice, sugar, and salt. Bring to a boil, reduce heat, and simmer, stirring often, until thickened and a spatula holds a trail when dragged across bottom of pan, about 4 minutes; stir in vanilla. Let cool.

Step 2

Do ahead: Filling can be made 4 days ahead. Cover and chill.