1/2 cup old-fashioned rolled oats
1 cup all-purpose flour (spooned and leveled)
1 cup whole-wheat flour (spooned and leveled)
3/4 teaspoon ground cinnamon
1/2 teaspoon baking soda
1/2 teaspoon salt
1 cup (2 sticks) unsalted butter, room temperature
3/4 cup light-brown sugar
1 large egg
8 ounces bittersweet or semisweet chocolate, cut into 30 squares
15 large marshmallows, halved horizontally
Preheat oven to 350 degrees. In a food processor, pulse oats until finely ground. Add flours, cinnamon, baking soda, and salt; pulse to combine. In a large bowl, using an electric mixer, beat butter and sugar until light and fluffy. Beat in egg, scraping down side of bowl. With mixer on low, beat in flour mixture just until combined.
Drop dough by tablespoons, 1 inch apart, onto two baking sheets. Top each with a chocolate square. Bake just until lightly golden, 11 to 13 minutes, rotating sheets halfway through. Remove sheets from oven; heat broiler. Top each cookie with a marshmallow. One sheet at a time, broil until marshmallows are lightly browned, 1 to 1 1/2 minutes. Transfer cookies to wire racks to cool.