I had wrist surgery a week and a half ago and have mostly been sitting on my couch at home since, attempting to get work done despite the challenge of a large, stiff cast and sore metacarpals. It's been slow going. Less slow is the rate of sugar I'm ingesting.
There are a couple keys to this cookie's gooey interior and soft and chewy texture. The first is the brown sugar itself. Unlike white sugar, brown sugar is slightly acidic, which means it readily reacts with the alkaline baking soda in the dough, creating plenty of carbon dioxide that softens the cookies as they bake. Brown sugar is also more hygroscopic than white sugar, meaning it hangs onto moisture more readily. (You can read more on how white and brown sugar differ in cookies here.)
The other key to these cookies is the butter. In recipes that call for creaming room temperature butter with sugar, the goal is to beat microscopic bubbles of air into the dough; when the baking soda activates, it's those little bubbles that puff up and expand into lofty cookies. In this recipe, though, the butter is melted first and then mixed with the brown sugar just enough to combine them, which means there's no aeration happening as there is with creaming. This balance of just the right amount of leavening from the brown sugar/baking soda reaction without the added leavening effects of creaming is key.
With less aeration, the brown sugar is left to excel at another of its jobs: its acidity speeds the development of gluten in the dough and the speed at which the proteins in the dough set during baking, enhancing the dense, chewy texture of the cookies.
To really play up the molasses-y flavor of the brown sugar, each dough ball is rolled in it first. This also enhances the crackly texture of the exterior of each cookie, along with an extra-intense blast of caramel flavor.