While dough is proofing, combine dark brown sugar, cinnamon, and salt in a small bowl.
Prepare a 9x9x3 pan by spraying the bottom and sides with nonstick baking spray.
Once your dough has doubled in size, chill in the refrigerator for 30 minutes.
At this point, you’ll want to work quickly, so your dough stays chilled and the filling doesn’t become messy as you roll and cut. Once chilled, set dough on a lightly floured work counter and roll into a 14x17 inch rectangle. (Make sure as you roll out your dough, you ensure it’s not sticking to the counter.) The long side of the rectangle should be facing you/even with your countertop.
Using your offset spatula, spread softened butter across the surface of the dough, leaving a 1-inch space at the bottom uncovered.
Sprinkle brown sugar mixture on the surface. Press lightly into the butter so it doesn’t fall out when you roll.
Starting from the top, quickly roll your dough into a log, careful to keep it even as you roll towards the bottom. Once your dough is completely rolled up, pinch the bottom uncovered edge into the dough, so it’s sealed. Position your log so the seal is on the bottom.
Using unflavored dental floss, trim ½ inch dough from both edges of your log. Then cut your log into 9 even pieces and place each piece cut side up in the prepared pan.
Cover the pan with either a clean kitchen towel or plastic wrap and place in a warm spot to rise for 40 minutes. The rolls should be almost doubled in size and look plump. Meanwhile, preheat the oven to 375 F.