Melt the butter in a small saucepan over low heat. Add the milk and keep on a low heat until warm to the touch. Remove from heat.

Add the warm milk and butter mixture to the bowl of a stand mixer with the dough hook attachment, along with the sourdough starter, egg, egg yolk, sugar, flour, and salt. Mix on low for about 5 minutes or until everything is fully combined. You may need to stop the mixer a couple of times to scrape the sides of the bowl. Cover the bowl and rest for 30 minutes.

After the dough has rested, knead on medium speed for 6-8 minutes until the dough is smooth, soft, and pulling away from the sides of the bowl. Do the windowpane test to confirm the gluten is properly developed before moving on.

Transfer to a lightly oiled bowl and cover with cling wrap or a damp cloth.

Bulk ferment. Ferment at room temperature for around 6 hours or until puffy and roughly doubled.

Make the filling. In a small bowl, mix together the brown sugar, cinnamon, vanilla extract, and cornstarch until fully combined. Set aside.

Lightly oil your surface, as well as your hands and the rolling pin. Turn the dough out onto the surface and roll horizontally first to get it to roughly 16" for 8 larger rolls or 24" for 12 smaller rolls, and then roll vertically to get your rectangle.

Spread the softened butter evenly across the entire surface, leaving a very small border at the top of the dough. Sprinkle the cinnamon sugar filling evenly over the butter and gently press it in with your hands.

Starting from the long edge closest to you, roll the dough up tightly into a log. Use unflavored dental floss or a sharp knife to cut into 8 large rolls or 12 smaller ones. Place cut-side up in a greased 9x13 pan.

Cover and let proof at room temperature for 3-4 hours until puffy and the rolls are touching in the pan.

Preheat oven to 350F (175C). Just before baking, pour the heavy cream evenly over the rolls. Bake for 20-25 minutes until golden on top and the centers are just set.

Let cool for 10-15 minutes before frosting.

Make the frosting. Beat the cream cheese and butter together until smooth and fluffy. Add the powdered sugar, vanilla, and salt and beat until combined. Add milk one tablespoon at a time until you reach a pourable but not watery consistency.

Pour the frosting generously over the warm rolls and let it melt down into all the crevices. Serve immediately.