Mix together ripe sourdough starter, all-purpose flour, granulated sugar and water. Knead the levain until it forms a cohesive ball. Set in a liquid measuring cup and cover for 10-12 hours until the levain has doubled in size and the top is rounded.

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, egg, 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 8-10 minutes until the dough is smooth. If mixing by hand, knead for 10-15 minutes.

Put the dough in a container and cover. Set the dough in a warm, 78-80ºF place for 4 hours. Take the temperature of the dough as needed to make sure it stays right around 78-80ºF. This temperature is optimal for fermentation. At the end of about 4 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, after the bulk fermentation has finished, cover the dough and put it in the refrigerator. Proceed with the recipe the next day, shaping and proofing the rolls before baking.

To a small saucepan add the blueberries, sugar and lemon juice. Set over medium heat and cook for about 5 minutes, mashing the berries during this time. Add cornstarch and whisk together over heat for about 10-15 seconds until thickened. Pour into a bowl and set aside to cool completely.

Right before you're ready to shape the rolls, pull the dough out of the refrigerator and mix together the lemon filling. Mix softened (or melty) butter, sugar, lemon zest and salt until it forms a thick paste.

Prepare a 9 by 13 pan with parchment paper or use a non-stick pan. 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 spread right to the edges of the dough. Add the blueberry compote on top of the lemon filling and spread to the edges. Starting with the dough closest to you, roll up the blueberry rolls and pinch together the seam. Flip the roll over, seam side down. Cut the long log of dough into 8 equal pieces. Place the rolls in the baking pan with two in the middle of the pan and three on each side of the pan, snuggling all 8 rolls together.

Cover the pan of rolls and let rise in an 80-85ºF place until puffed up and risen, about 3-4 hours. Do not bake these rolls if they have not risen.

Pre-heat the oven to 350ºF. Bake the blueberry rolls for about 30 minutes until baked all the way through. Check the middle of a few of the rolls by using a knife to pry up a bit of the roll and make sure it is baked to your liking.

Using a hand mixer, 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 frosting over the tops of the blueberry lemon sweet rolls while they are still warm. Enjoy!