Make the Dough: In a large mixing bowl, whisk together flour, baking soda, baking powder, and salt.
In another bowl, combine sourdough discard, melted butter, egg, warm milk, and maple syrup.
Add wet to dry: Mixing with an electric mixer fitted with a dough hook on low speed until combined. Increase to medium speed and knead until a soft, slightly sticky dough forms. If needed, sprinkle in a cup of flour a little at a time during the kneading process for the best results.
First Rest: Cover bowl with towel and let dough rest for 10 minutes.
Roll & Fill: On a lightly floured surface, roll dough into a rectangle about 10×29″. Spread the maple mixture of softened butter, maple syrup, brown sugar, granulated sugar, and cinnamon over the dough.
Roll tightly into an 18-inch-long log. Using a sharp knife, cut into about 25 mini rolls.
Bake: Arrange rolls on a parchment-lined baking dish. Cover loosely with aluminum foil and bake at 330°F (or 350°F if your oven runs cool) for 22–25 minutes, until golden brown.
Maple Finish: Brush hot rolls with maple syrup. Allow to cool slightly.
Frosting: Beat cream cheese, softened butter, powdered sugar, maple syrup, and salt with a whisk attachment on medium-high speed until smooth and creamy. Spread generously over slightly cooled rolls.