Make the Levain: Mix together the ripe sourdough starter, all-purpose flour, granulated sugar, and water. Knead until you have a cohesive dough. Transfer to a measuring cup, cover, and let it rest for 10–12 hours at 78ºF until doubled with a domed top.

Prepare the Dough: Warm the milk until it reaches 90–100ºF. In a stand mixer bowl with the dough hook attached, add the ripe levain, warm milk, melted butter, granulated sugar, egg, salt, lemon zest, and most of the bread flour. Knead for several minutes, adding additional flour as needed. Continue kneading for 8–10 minutes total, or up to 15 minutes by hand, until the dough is smooth and tacky but not sticky. It should mostly pull away from the bowl.

First Rise (Bulk Fermentation): Place the dough in a covered container and let rise in a warm spot (78–80ºF) for about 4 hours. Check the dough to ensure it feels elastic and is puffed up; if not, let ferment for another 30 minutes and check again.

Chill Overnight: Once bulk fermentation is complete, cover the dough and refrigerate overnight. This step allows you to time your baking for the next day.

Make the Blueberry Compote: Combine blueberries, granulated sugar, and lemon juice in a small saucepan. Cook over medium heat for around 5 minutes, mashing the berries with a spoon. Stir in the cornstarch and cook for 10–15 seconds more until it thickens. Pour into a bowl and let cool completely.

Mix Lemon Filling: When ready to shape, mix together the softened (or melted) butter, granulated sugar, lemon zest, and a pinch of salt until a thick paste forms.

Shape the Rolls: Line a 9x13-inch pan with parchment paper or lightly grease. Roll out the dough to approximately a 16x12-inch rectangle. Evenly spread the lemon filling over the surface, reaching all edges. Top with the cooled blueberry compote, spreading it to the edges as well. Roll up the dough, starting from the edge closest to you, and pinch the seam closed. Place the roll seam-side down, then slice into 8 equal pieces. Arrange rolls in the prepared pan, with two in the center and three on each side.

Final Proof: Cover the pan and let the rolls rise in a warm spot (80–85ºF) for 3–4 hours, or until visibly puffed and risen. Don't bake until fully proofed.

Bake: Preheat the oven to 350ºF. Bake the rolls for about 30 minutes, or until the centers are fully baked when checked with a knife. If the center appears underdone, return to the oven for a few minutes more.

Prepare the Frosting: Beat softened butter and cream cheese together using a hand mixer. Add powdered sugar, lemon juice, lemon zest, and a pinch of salt. Whip for 2–3 minutes until light and fluffy, scraping down the sides of the bowl as needed.

Frost and Serve: While the rolls are still warm, spread the frosting over the tops. Enjoy fresh.