Prepare the Dough: In a large bowl, mix warm milk, sourdough starter, sugar, and egg until combined. Gradually add bread flour and salt, then knead until a dough forms. Add butter in small pieces, kneading until fully incorporated. Cover and let rest overnight in the fridge.
Prepare the Filling: In a saucepan, combine frozen raspberries, sugar, and cornstarch. Cook over medium heat until thickened, about 5 minutes. Stir in lemon zest and let cool.
Roll & Fill: On a floured surface, roll out the dough into a rectangle. Spread the raspberry filling evenly over the surface. Roll up tightly and cut into 12 equal rolls.
Second Rise & Bake: Place rolls in a greased baking dish, cover, and let them rise for 2 hours at room temperature. Bake at 375°F (190°C) for 25-30 minutes until golden brown.
Make the Frosting & Serve: Beat together cream cheese, butter, powdered sugar, lemon juice, and milk until smooth. Spread over warm rolls and enjoy!
