In a large mixing bowl, combine the all-purpose flour, granulated sugar, and instant yeast. Gradually add lukewarm milk, softened butter, egg, salt, and lemon zest to the dry ingredients. Mix until combined, then knead the dough for about 15 minutes until smooth and elastic. Cover and let rise for 1 hour or until doubled in size.

Prepare the filling by combining milk, poppy seeds, and sugar in a saucepan. Bring to a gentle boil, reduce heat, and simmer for about 5 minutes until thickened. Stir in lemon zest and allow to cool.

After the dough has risen, punch it down and roll it out into a large rectangle. Spread a layer of spreadable butter over the dough, followed by half of the cooled filling. Fold the dough in half, seal edges, roll it out again, and spread the remaining filling. Roll tightly and cut into equal pieces.

Arrange the sliced pieces upright on lined baking sheets, leaving space between them. Indent the tops with a wooden spoon handle. Cover with a towel and let rise for 30 minutes.

Preheat your oven to 350°F (or 325°F for convection). Bake for about 20 minutes or until golden brown and a toothpick comes out clean.

Prepare the glaze by mixing powdered sugar and lemon juice until smooth. Brush the warm buns with the glaze for a shiny finish and allow to cool slightly before serving.