Preheat the oven to 340°F and line an 8x8 inch baking pan with parchment, leaving an overhand on all sides.
Make the cookie dough:
Add the flour, oats, soda and salt to a small bowl and mix well. Set aside.
In a separate bowl, whisk the softened butter until creamy. Add the sugar and whisk for a minute or two. Add the egg and keep whisking until it's well incorporated. Finally, add the vanilla and mix it in until smooth.
Add the dry ingredients to the wet and fold with a rubber spatula until well combined, and no more dry oats are left.
Assemble the bars:
Press 2/3 of the cookie dough into the bottom of the prepared baking pan.
Evenly sprinkle most of the chopped chocolate over the base, then add the marshmallows on top. Press down lightly.
Scoop the remaining cookie dough with a small cookie scoop. Flatten with your hands and evenly add on top of the marshmallow layer. You don't need to cover the entire top, but make sure your blobs of dough are evenly distributed.
Bake the bars:
Bake the bars for around 25 minutes, or until the oatmeal cookie dough is set and the top is golden brown. They will still be gooey and very soft, that's fine!
Cool the bars in the pan for at least 30 minutes before lifting them out to slice.