Activate yeast: Mix lukewarm milk, 1 tsp sugar, and yeast in a small bowl. Wait 5–10 minutes until frothy.
Make dough: In a large bowl, combine flour, salt, sugar, and lemon zest. Cut in butter until crumbly. Add eggs and yeast mixture. Knead until smooth and elastic.
Rest dough: Cover and let rise for 1 hour or until doubled in size.
Prepare filling: Heat milk gently in a saucepan. Add sugar, honey, and salt. Stir in poppy seeds and cook 5–7 minutes until thick. Remove from heat, add vanilla, and cool slightly.
Roll dough: Punch down dough, roll into a rectangle about 1/4 inch thick. Spread filling evenly, leaving 1/2 inch border.
Form roll: Roll from long side into a log. Seal edges and place seam-side down on a baking sheet. Brush with beaten egg.
Bake: Preheat oven to 350°F (175°C). Bake 35–45 minutes until golden.
Cool and slice: Let cool for at least 30 minutes. Slice 1/2–3/4 inch thick and serve.