The recipe online recommended a fork or electric mixer to blend them together well. I tried the electric mixer (use A DEEP bowl!), but it wasn't quite doing the job. I found clean, dry hands worked best, and I just sort of smeared the sugar and molasses together repeatedly - basically the same motion I use when adding butter to flour for pie crust.
And a perk of doing this was the lightest, fluffiest brown sugar that was so easy to work with! I measured what I needed into a bowl, whisked in the cinnamon, and found it super easy to evenly distribute the mixture over the melted butter I'd smeared and let cool on the dough. It used to be I'd melt the butter, sugar and cinnamon together for a smooth paste, but it wasn't always easy to spread on the soft dough.
Mmmm dough! Smells so good!
Now I do feel this batch is a bit more grainy on the inside. I don't know if the granulated sugar I used is more coarse than whatever is used to make brown sugar (maybe try the metal food processor blade to grind the sugar and combine with the molasses?), or if it's because I did a double batch of filing?
I should make cinnamon rolls again soon, with a single batch of filling and a single batch of frosting (double is just too much), and make my own brown sugar again.
We had some leftovers, so I've wrapped each in plastic wrap (one in foil) and frozen them - will see how they recover later. Also patted the frosting out on plastic wrap to freeze, and will plan to break off some even I reheat a roll.
Daisy is looking forward to more cinnamon rolls. I told her it would be rude for me to only offer her the last coil of cinnamon roll.