Preheat your oven to 200°F (95°C). In a large bowl, whisk together flour, yeast, poppy seeds, and fine salt. Warm buttermilk with sugar and butter until melted, then mix with dry ingredients until a sticky dough forms.

Add the eggs one by one, mixing after each addition. Incorporate lemon zest until dough is elastic and smooth. Gradually add more flour until the dough is tacky but not sticky. Cover and let rest for 10-20 minutes.

In a medium bowl, mix granulated sugar, light brown sugar, and lemon zest. Massage zest into sugar, add softened butter, and whisk until spreadable.

On a floured surface, roll out dough into a 12"x18" rectangle. Spread filling evenly, roll tightly into a log, and cut into 12 pieces. Arrange in a greased 9"x13" pan, cover, and allow to rise in warm oven for 30 minutes.

Preheat oven to 350°F (175°C) and bake for 28-32 minutes until golden brown. Allow to cool slightly in the pan.

For icing, whisk powdered sugar, lemon juice, and melted butter until smooth. Spoon over cooled rolls.