Preheat the oven to 425 degrees F. Line 2 baking sheets with silicon mats or parchment paper and set aside.
For the cookies: Beat the butter and sugar with a hand mixer until light and fluffy. Add the egg and vanilla until just incorporated. Scrape down the sides of the bowl.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, salt and baking powder.
In a third bowl, combine the milk and sour cream.
Add half of the milk mixture to butter mixture while mixing on low speed. Then add half of the flour mixture and mix until incorporated. Repeat the process.
With a measured scoop, portion out the batter into small rounds on the prepared baking sheets, spaced at least an inch apart. You can make the cookies whatever size you like; I like mine a bit big so I use a 4-ounce ice cream scoop.
Bake until the tops lightly spring back when touched, 8 to 10 minutes. Remove from the oven and allow to cool completely.
For the filling: Add the sliced strawberries to a bowl and sprinkle with the granulated sugar and let sit for 15 minutes.
Preheat the oven to a broil and set a rack 6 inches from the broiler.
When the cookies are cooled, place them round-side down and then top half of them with sliced strawberries.
Press 3 marshmallows together using the palm of your hand, until mostly flattened, and then place them on the other half of the cookies.
Place the marshmallow-covered cookies back on the baking sheets and place under the broiler to let the marshmallows char and soften until lightly golden on top, 1 to 4 minutes. Keep an eye on them as they can burn quickly.
For assembly: Top the strawberry side of the cookie with one of the marshmallow cookies, gently press together and then stand back and marvel at your creation.