Mix together ripe sourdough starter, all purpose flour, granulated sugar and water. Knead the levain until it forms a cohesive ball. Place in a clear jar and set in a warm 78ºF place. Cover and let rise for 10-12 hours until the levain has doubled in size and the top is rounded.

Mix the Dough: Warm the milk in the microwave (about 1 ½ minutes full power) or on the stove. It should be around 90-100ºF, no warmer than that. To the bowl of a stand mixer with a dough hook, add the ripe levain, warmed milk and melted butter.

Add the sugar, eggs, salt, lemon zest and most of the bread flour. Turn on the dough hook and knead for a few minutes. Add the remainder of the bread flour as needed. The dough should be tacky, not overly sticky and should all cling to one side of the bowl (or away from the edges depending on your mixer). Knead for a total of 5-7 minutes until the dough is smooth.Alternatively you can knead this dough by hand if you don't have a stand mixer.

Bulk Fermentation: Put the dough in a container and cover. Set the dough in a warm, 78-80ºF place for 4-5 hours. Take the temperature of the dough as needed to make sure the dough temperature stays right around 78-80ºF. This temperature is optimal for fermentation. At the end of about 4-5 hours, the dough should be puffed up and feel very elastic. If it doesn't feel this way, let it bulk ferment for another half hour and check again.At this point you can refrigerate the dough overnight, shaping and proofing the rolls the following morning if desired.

Mix up the Filling: Near the end of bulk fermentation, mix up the lemon filling. To a small bowl, add the softened butter, granulated sugar, lemon zest and salt. Mix together. Set aside.

Shaping: Line a 9 by 13 pan with parchment paper. Roll the dough out into an approximate 16 by 12 rectangle. Spread the lemon filling all over the dough with your fingers, making sure to cover up to the edges of the dough. Starting with the dough closest to you, roll up and pinch together the seam. Flip the lemon roll over, seam side down. Cut the long log of roll dough into 12 equal pieces. Place the rolls in the parchment lined pan in a 3 by 4 grid.

Proofing: Cover the pan and let rise in a 78-80ºF place until puffed up, doubled in size and the dough feels light and airy, about 6-8 hours. Do not bake these rolls if they have not risen. At this point the rolls can be baked right away OR for morning lemon rolls, cover them and stick in the refrigerator overnight to be baked straight from the fridge the next morning.

Baking: Preheat the oven to 350º F. Bake for about 25-30 minutes until the center of the lemon rolls is 190-195ºF and they are baked all the way through. Remove from the oven and let rest for about 5 minutes.

Lemon Cream Cheese Icing: Use a hand mixer to whip together softened butter and cream cheese. Add the powdered sugar, lemon juice, lemon zest, and salt. Whip together for 2-3 minutes, scraping down the sides of the bowl as needed. The frosting should be light and fluffy.

Spread icing over the tops of the lemon rolls while they are still warm and enjoy.