Preheat oven to 350F. Grease a 10-inch cast iron skillet with butter.
In a small bowl, combine the dry ingredients. Set aside.
In a large bowl, beat the butter, sugars, eggs, and vanilla until smooth. Add the molasses and mix until just combined. Gradually beat in the dry mixture. Stir in the oats.
Press the dough into the prepared skillet. Bake for 22 minutes, then check. Make sure the edges are set, but the center still looks a tad gooey. If need be, continue to bake in 2 minute increments. Once done, let cookie cool for 15-20 minutes before frosting.
While the cookie is cooling, make the creme topping. In a medium mixing bowl, beat the butter until smooth. Add in the powdered sugar and mix. Next, beat in the heavy cream, vanilla, and salt on high, until the mixture is smooth.
Once the cookie is cooled, evenly spread the frosting on top. Please note, the butter may slightly separate when topped on a warm cookie. It doesn't effect the flavor, but does affect the presentation. If you don't want this to happen, let the cookie completely cool before frosting. Enjoy!