Roll out your dough into a rectangle. Spread the butter evenly across the dough. Mix together the granulated sugar and cinnamon until well combined. Sprinkle evenly across the butter. Crumble the brown sugar evenly across the whole thing. See pictures above for a visual reference.
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Notes
This makes enough filling for 12 standard cinnamon rolls, or a batch made in into a 9x13 pan.
This recipe can easily be doubled or tripled if needed.
The butter works so much better when it's super soft. If you are working in a warm kitchen (about 74° F or higher) the butter will be fine at room temperature. If you are working in a cooler kitchen, place in the microwave in 4 second intervals, until it is very soft and about to melt (but not melted).
To keep your filling from leaking out of the roll: After slicing your rolled up log of dough into rounds, place your rolls on your work space, swirl sides up and down. Loosen up your roll so it isn't too tight, then grab the last inch or so of the swirl and spread the dough out into a thin circular shape. Wrap it around the top of the roll to completely cover the swirl and pinch the edges together. This will keep all that ooey gooey goodness inside the roll. See pictures above for a visual guide.