Brown the Butter for the Cookies: Add the 21 Tbsp (294g) butter to a saucepan over medium heat. Cook until it foams, then turns golden with brown speckles and smells nutty (about 4–6 minutes). Pour into a mixing bowl and let it cool 20–30 minutes, until just slightly warm, not hot.

Prep the Oats: Add the old-fashioned oats to a blender or food processor. Pulse to a fine, sandy flour.

Make the Cookie Dough: Preheat oven to 350°F (175°C). Line baking sheets with parchment. To the cooled brown butter, add brown sugar, granulated sugar, and molasses. Stir until smooth. Add eggs and vanilla; whisk until glossy. In a separate bowl, whisk together the flour, quick oats, blended oats, cornstarch, baking soda, baking powder, salt, and spices. Add dry ingredients to wet and mix until just combined. Let it rest 10 minutes so the oats can hydrate and the dough can thicken.

Scoop & Bake: Scoop 35g (1.5 Tbsp) portions onto the sheet, 2–3 inches apart. Bake 8–10 minutes until the edges look set but the centers still look soft. Right when they come out of the oven, use a large round cookie cutter or the rim of a glass to scoot them into perfect circles. Cool completely.

Make the Brown Butter Marshmallow Filling: Brown ½ cup (113g) butter the same way as for the cookies. Pour into a bowl and chill in the fridge until it’s like softened butter, spreadable, not melted and not firm (this takes about 1 hour). Beat the cooled browned butter 2 minutes until creamy and pale. Add marshmallow fluff, vanilla, and salt; mix until smooth. Add powdered sugar and beat 2–3 minutes until fluffy. Add 1–1½ Tbsp cream, just until it’s smooth and pipeable.

Assemble: Pair cookies by size. Pipe or spread a thin, even layer of filling. Sandwich and gently press so the filling reaches the edges. Chill 15–20 minutes for that true Little Debbie bend-and-chew texture.