In a large liquid measuring cup, measure 2 and 1/2 cups warm water. Warm water for this recipe is between 110 degrees and 120 degrees F.
Pour in 4 and 1/2 teaspoon active dry yeast. That's 2 packets if you purchase the packs.
Let it stand for 10 minutes to activate and foam.
Once yeast is active pour it into the bowl of a stand mixer. (It should be foamy and bubbly on top. If it's not, test the yeast and test the water temperature, read my post How to cook with yeast (link below) for questions and troubleshooting.)
Add cake mix, salt, flour, and vanilla.
Using the dough hook attachment, knead the dough for 1 to 2 minutes. Dough should pull away from the bowl and be a little sticky.
Spray a large bowl with non-stick vegetable spray and place dough in it to rise. Cover with plastic wrap that has also been sprayed with non-stick vegetable spray. Place bowl in a warm, non-drafty place like your oven (that's not on) or microwave.
Let sit for 45 to 75 minutes until id is doubled in size. (Temperature and humidity will cause it to rise differently so just keep an eye on it.)
Punch the dough down and knead a couple of times, place the dough back in the bowl, cover with plastic wrap, and allow to rise a second time until it is doubled in size again.
Once the dough has gone through the second rise, remove it and place on a clean, floured surface.
Knead the dough 3 to 4 minutes with well-floured hands. Roll the dough into a large rectangle. Roll it large enough so the dough is approximately 1/2 inch thick. Eyeball it or measure if necessary.
At this point, you'll want to preheat the oven to 350 degrees F.
For the filling - smear the softened butter over the top of the dough, carefully with a butter knife or your fingertips.
Next, sprinkle brown sugar over butter. Spread with hands to get it even.
Sprinkle ground cinnamon over brown sugar evenly. *If you're putting sprinkles in the filling sprinkle them evenly over the cinnamon now.
Starting on the long side, roll tightly to form a long, skinny log.
Slice into even rolls. I usually get 18 to 20 rolls.
Place the rolls in a greased 9x13 baking pan. 12 rolls will fit in a 9x13 inch pan. Place the remaining in another 9x13 pan if you have 12 more. Or place 6 rolls each in 3 round 9-inch square or round baking pans.
Regardless of the pan you choose, it will take the rolls 18 to 22 minutes to bake. You want the top to be browned lightly. You may have to center the center of the center roll for doneness. Cook at 2-minute increments until done in the center.
While cinnamon rolls are baking make the cream cheese frosting (recipe here)
Allow cinnamon rolls to cool 3 to 5 minutes before frosting.
Add sprinkles to the top of cream cheese frosting if desired.