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1. In a large mixing bowl, combine the sourdough discard and warm milk. Stir until well mixed.
2. Add the granulated sugar, melted butter, egg, and vanilla extract to the bowl. Whisk everything together until smooth and creamy.
3. In a separate bowl, mix together 3 1/2 cups of all-purpose flour, salt, and instant yeast. Gradually add this dry mixture to the wet ingredients, stirring with a wooden spoon or spatula until a shaggy dough forms.
4. If the dough is too sticky, gradually add more flour, a tablespoon at a time, until it becomes manageable.
5. Once the dough comes together, turn it out onto a lightly floured surface. Knead the dough for about 5-7 minutes until it is smooth and elastic.
6. Form the dough into a ball and place it in a greased bowl. Cover it with a clean kitchen towel or plastic wrap and let it rise in a warm place for about 1-2 hours, or until it has doubled in size.
7. While the dough is rising, prepare the filling. In a small bowl, mix together the brown sugar and ground cinnamon until well combined.
8. Take the softened butter and spread it evenly over the surface of the dough once it has risen.
9. Sprinkle the cinnamon-sugar mixture evenly over the buttered dough, making sure to cover the entire surface.
10. Once the dough has risen, punch it down gently to release the air. Turn it out onto a lightly floured surface.
11. Roll the dough into a rectangle, approximately 12×18 inches in size.
12. After rolling out the dough, spread the softened butter over the entire surface, leaving a small border around the edges.
13. Sprinkle the cinnamon-sugar mixture evenly over the buttered surface.
14. Starting from the long edge, carefully roll the dough into a tight log. Pinch the seam to seal it once you reach the end.
15. Using a sharp knife or a piece of dental floss, slice the log into 12 equal pieces.
16. Place the sliced rolls into a greased 9×13 inch baking dish, leaving a little space between each roll for them to expand.
17. Cover the dish with a kitchen towel or plastic wrap and let the rolls rise again in a warm place for about 30-45 minutes, or until they have puffed up and are touching each other.
18. Preheat your oven to 350°F (175°C) while the rolls are rising.
19. Once the rolls have risen, place the baking dish in the preheated oven and bake for 25-30 minutes, or until golden brown on top.
20. While the rolls are baking, prepare the icing by whisking together the powdered sugar and milk in a small bowl until smooth.
21. Once the rolls are done baking, remove them from the oven and let them cool for a few minutes before drizzling the icing over the top.

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