Preheat oven to 350F. Line an 8-by-8-inch baking pan with aluminum foil, spray with cooking spray; set aside.
In a large, microwave-safe bowl melt the butter, about 1 minute on high power.
Wait momentarily before adding the egg so you don’t scramble it. Add the egg, brown sugar, molasses, vanilla, salt, cinnamon, and whisk until smooth.
Add the oats and stir to combine.
Add the flour and stir until just combined, don’t overmix.
Turn batter out into prepared pan, smoothing the top lightly with a spatula. Bake for about 24 to 25 minutes, or until done. A toothpick inserted in the center should come out clean, or with a few moist crumbs, but no batter.
Place pan on top of a wire rack to cool completely before frosting. Don’t frost warm bars or the frosting will melt.
Frosting
To the bowl of a stand mixer fitted with the paddle attachment (or large bowl and electric hand mixer), add the butter, confectioners’ sugar, and beat on medium-high speed (start on low so you don’t spray sugar everywhere) until light and fluffy, about 3 minutes.
Stop, scrape down the sides of the bowl, and add the marshmallow creme, salt, and beat on medium-high speed until well combined, about 1 minute.
Turn frosting out over bars and lightly smooth it with a spatula or offset knife.
Notes
Storage: Bars will keep airtight at room temperature for up to 5 days. I’m comfortable storing buttercream-style frosting at room temp but if you’re not, refrigerate the bars noting they’ll be more prone to drying out in the fridge.