While the dough is proofing, combine the dark brown sugar, cinnamon, and salt in a small bowl and mix together.
Prepare a 9x13 inch 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 the dough on a lightly floured 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/parallel to the edge of the countertop.
Using your offset spatula, spread softened butter across the surface of the dough, leaving a 1-inch space at the bottom uncovered.
Sprinkle the brown sugar mixture on the surface. Press lightly into the butter, so it doesn’t fall out when you roll it.
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 of dough from both edges of your log. Then cut your log into nine even pieces and place each piece cut side up in your prepared pan.
Cover the pan with a clean kitchen towel or plastic wrap and return to the 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.