In a large mixing bowl, combine 4 cups of all-purpose flour, 1/4 cup of granulated sugar, and 2 teaspoons of instant yeast. Gradually add 1 cup of lukewarm milk, 1/4 cup of softened butter, 1 egg, 1/2 teaspoon of salt, and 2 teaspoons of lemon zest. Mix until a dough forms, then knead for about 15 minutes until smooth and elastic. Cover with a damp cloth and let rise in a warm place for about 1 hour, or until doubled in size.

While the dough rises, prepare your poppy seed filling. In a saucepan over medium heat, combine 1/2 cup of milk, 1/3 cup of poppy seeds, and 1/4 cup of sugar. Bring to a gentle boil, then reduce heat and simmer for about 5 minutes, stirring until the milk is absorbed. Once thickened, remove from heat, stir in the remaining 2 teaspoons of lemon zest, and allow the mixture to cool to room temperature.

Once the dough has risen, punch it down and roll it out on a lightly floured surface into a rectangle about 1/4 inch thick. Spread 2 tablespoons of softened butter over the dough, followed by half of the cooled poppy seed filling. Fold the dough like a letter, then roll it out again to the same thickness. Spread the remaining filling, roll it tightly, and slice into pieces every 4 cm to form your buns.

Line two baking sheets with parchment paper and arrange the sliced buns upright, ensuring they have some space to expand. Cover the trays loosely with a clean kitchen towel and allow the buns to rise in a warm area for about 30 minutes, or until puffy and slightly swollen.

Preheat your oven to 350°F (or 325°F for convection ovens). Once heated, place the baking sheets in the oven and bake for around 20 minutes, or until the tops are golden and the buns sound hollow when tapped lightly. Keep an eye on them to avoid overbaking.

As the buns bake, prepare the glaze by mixing 1 cup of powdered sugar with the juice of one lemon until smooth. After removing the buns from the oven, let them cool for about 5-10 minutes before brushing or drizzling the glaze generously over the warm Lemon Mohn Franzbrötchen to allow it to set beautifully.