Prepare the Dough Base: Preheat your oven to 200 F (95 C). In the bowl of a stand mixer or a large mixing bowl, combine 4 1/4 cups of all-purpose flour, the instant dry yeast, poppy seeds, and salt.

Heat the Wet Ingredients: In a separate microwave-safe bowl, combine the buttermilk, 1/2 cup granulated sugar, and 1/2 cup butter. Heat in the microwave for 1 minute, then stir. The butter should be mostly melted and the mixture should be warm to the touch, but not hot enough to kill the yeast.

Mix the Dough: Slowly pour the warm buttermilk mixture into the dry ingredients. Mix on medium speed using a dough hook or a large wooden spoon. Add the eggs one at a time, mixing on medium-low until a sticky ball forms. Add 2 tablespoons of lemon zest and continue mixing for a few minutes to develop elasticity.

Adjust Flour and Rest: Add additional flour 1 tablespoon at a time until the dough clears the sides of the bowl but still feels tacky. Cover the bowl with plastic wrap and let the dough rest for 10 to 20 minutes to relax the gluten.

Prepare the Lemon Filling: In a medium bowl, combine the granulated sugar, light brown sugar, and lemon zest. Use your fingertips to massage the zest into the sugar until fragrant. Whisk in the softened butter until combined.

Roll and Fill: On a lightly floured surface, roll the dough into a 12-inch by 18-inch rectangle (about 1/2 cm thick). Spread the lemon filling evenly over the dough, leaving a 1/2-inch border on one long side.

Shape the Rolls: Roll the dough tightly starting from the long side with filling. Use dental floss to cut the log into 12 equal pieces. Place the rolls into a greased 9x13-inch baking pan.

The Assisted Rise: Turn the oven off. Cover the pan with aluminum foil and place it in the warm (but turned off) oven for 30 minutes. The rolls should look puffy and increased in size.

Bake: Remove the rolls from the oven and discard the foil. Turn the oven back on to 350 F (175 C). Once preheated, bake the rolls for 28 to 32 minutes until they reach a light golden brown color.

Ice and Serve: While the rolls cool for 10 to 20 minutes, whisk together the powdered sugar, lemon juice, and melted butter. Spoon the icing over the warm rolls and garnish with extra lemon zest.