Step 1

Preheat oven to 350 degrees (you want the oven ready as soon as you're done scooping!)

Step 2

In a small microwave-safe bowl, add the butter and cover with a paper towel (will help for splattering). Melt the butter for 30-45 seconds until all is melted (no chunks left).

Step 3

Pour butter into a larger bowl, then add in brown sugar and powdered sugar 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).

Step 4

Add in egg and vanilla, then mix. Next, add in baking soda, cinnamon, salt, cornstarch, and cocoa powder and mix again.

Step 5

In a food processor or blender, add quick oats and raisins and pulse until broken up (you want them coarse, not a fine powder). I did this for 10 pulses in my Ninja food processor.

Step 6

Add oatmeal raisin mixture and flour to dough, then mix until all the flour disappears into the dough. Add milk and mix until combined.

Step 7

Line a baking sheet with parchment paper.

Step 8

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.

Step 9

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.

Step 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.

Step 11

Beat the softened butter 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. You may need to stop and scrape down sides. Add vanilla and salt and turn mixer to medium speed. Add in marshmallow fluff 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.