Toward the end of rising time, mix the brown sugar, cinnamon, and cornstarch in a bowl. Maybe you are asking why cornstarch? It makes the innards OH SO GOOEY, without letting them run out all over the place and burn in the bottom of the pan. In theory, you could use arrowroot or xanthan gum. If you have them on hand, by all means, but I find arrowroot tends to create more of a slimy texture.