Preheat oven to 350?F. Line an 8-by-8-inch baking pan with aluminum foil and spray with cooking spray; set aside.

In a large, microwave-safe bowl, melt the butter for about 1 minute on high power.

Wait momentarily before adding the egg. Add the egg, brown sugar, molasses, vanilla, salt, and cinnamon, and whisk until smooth.

Add the oats and stir to combine.

Add the flour and stir until just combined, being careful not to overmix.

Turn the batter out into the prepared pan, smoothing the top lightly with a spatula. Bake for about 24 to 25 minutes, or until done.

Place the pan on a wire rack to cool completely before frosting.

In a stand mixer bowl (or large bowl with an electric hand mixer), combine the butter and confectioners? sugar, and beat on medium-high speed until light and fluffy, about 3 minutes.

Scrape down the sides of the bowl, then add the marshmallow creme and salt. Beat on medium-high speed until well combined, about 1 minute.

Turn frosting out over cooled bars and lightly smooth it with a spatula.