Preheat the oven to 350 degrees.
Line a 9x13 pan with parchment paper; be sure to add enough to have a 1" overhang on each side for easy bar removal.
Whisk together your dry ingredients in a small bowl- the flour, cinnamon, baking soda, salt, and oats. Set aside.
In a stand mixer fitted with a paddle attachment, mix together the butter and all of the sugars. Allow to cream nicely, about 3 minutes.
Next, add your egg, molasses, and vanilla and mix until thoroughly incorporated.
With your mixer on the lowest speed slowly add in the dry ingredients. Once mostly incorporated, you should raise the speed on your mixer. If the bottom of the mixer is not mixed in, grab a wooden spoon and mix until combined.
Pour the oat batter into the 9x13 pan. Bake at 350 degrees for 25-30 minutes.
During the last 5 minutes of baking, melt the 9 ounces of milk chocolate and 3/4 cup of smooth peanut butter. Stir until fully combined and melted.
Remove the cookie bar from the pan and pour the chocolate peanut butter mix on top. Place in the fridge until cooled and then slice and serve!