Design Eat Repeat

0 Preheat oven to 350℉ (you want the oven ready as soon as you're done scooping!)
2 In a small microwave-safe bowl, add the unsalted butter ([adjustable]½ cup, i.e. 1 stick[/adjustable]) and cover with a paper towel (will help for splattering). Melt the butter for 30-45 seconds until all is melted (no chunks left).
3 Pour melted unsalted butter into a larger bowl, then add in light brown sugar ([adjustable]¾ cup) and powdered sugar (½ cup[/adjustable]) and use a spatula to stir until combined (it won't look totally smooth). Allow the mixture to cool for a few minutes until no longer hot (you don't want to cook the egg in the next step).
8 Add in egg ([adjustable]1 large) and vanilla extract (1 teaspoon), then mix. Next, add in baking soda (¼ teaspoon), cinnamon (1 teaspoon), salt (½ teaspoon), cornstarch (1 tablespoon), and unsweetened cocoa powder (1 teaspoon[/adjustable]) and mix again.
9 In a food processor or blender, add quick oats ([adjustable]1 cup[/adjustable]) and raisins ([adjustable]½ cup[/adjustable]) and pulse until broken up (you want them coarse, not a fine powder). I did this for 10 pulses in my Ninja food processor.
4 Add oatmeal raisin mixture and all-purpose flour ([adjustable]1 cup[/adjustable]) to the dough, then mix until all the flour disappears into the dough. Add milk ([adjustable]2 tablespoons[/adjustable]) and mix until combined.
5 Line a baking sheet with parchment paper. 
13 Regular Sized: Scoop dough into 2-tablespoon sized scoop, roll into balls, then bake at 350℉ for 9-10 minutes. You want them look slightly underbaked, as they will continue baking on the pan (my oven was perfect at 9 minutes). This is important for a soft cookie!Minis: Scoop into a heaping 1-tablespoon scoop, split in half, then roll into balls. Bake at 350℉ for 5-6 minutes.
7 Remove from oven and (if cookies have a slight puff to them) gently tap pan on counter to "deflate" cookies. Allow the cookies to cool on the hot pan for 10-15 minutes before removing them. This will allow them to finish baking on the hot pan.
10 Once cooled, use a knife or piping bag to pipe frosting in the center of one cookie then press another one on top to create a sandwich. If piping, don't pipe all the way to the edge to ensure the frosting doesn't squeeze out.
To Make Filling:
12 Beat the unsalted softened butter ([adjustable]½ cup, i.e. 1 stick[/adjustable]) on medium-high speed for 2 minutes until smooth. Turn the mixer to low speed then add the powdered sugar 1/4 cup at a time ([adjustable]1 cup[/adjustable] total). You may need to stop and scrape down the sides. Add vanilla extract ([adjustable]½ teaspoon) and salt (¼ teaspoon[/adjustable]) and turn the mixer to medium speed. Add in marshmallow fluff ([adjustable]7 ounces[/adjustable]) and beat on high for 2-3 minutes until creamy. If it looks too runny, add more powdered sugar. If it looks too stiff, add a tiny bit of milk until you get to the desired consistency. Spread onto cooled cookies.