Preheat oven to 350 degrees F (177 degrees C) and lightly grease a 9×9 inch glass baking pan.

Pulse the rolled oats in a food processor until they become a fine powder.

Whisk the flours, oat powder, brown sugar, baking soda, salt, and cinnamon together in a large bowl.

Cut the chilled butter into the dry mixture and work it in until the mixture has a coarse meal texture.

Whisk together the milk, vanilla extract, and maple syrup in a small bowl. Add the liquid mixture to the dry ingredients and combine until just mixed. Knead briefly until the dough comes together, then handle with a floured surface, floured hands, or a sturdy spoon as needed because the dough will be sticky.

Press the dough evenly into the prepared 9×9 inch glass baking pan.

Bake for 15 to 20 minutes, until a toothpick inserted into the bar comes out clean while the center remains slightly soft.

Refrigerate the baked bars for a few hours until the base firms and can be cut cleanly.

Cut the chilled graham bars and place chocolate pieces and one marshmallow on each bar.

Broil the topped bars on the high broil setting for about 3 minutes, checking frequently, until the marshmallows are lightly golden. Remove from the oven and enjoy.