Preheat oven to 425 degrees F (220 degrees C). Line baking sheets with parchment paper.

Beat softened butter and light brown sugar until light and fluffy.

Add egg and vanilla extract, mixing until just incorporated.

Scrape down the sides of the bowl to ensure even mixing.

In a separate bowl, whisk together flour, Dutch process cocoa powder, baking soda, salt, and baking powder.

In another bowl, combine milk and sour cream.

Add half the milk mixture to the butter mixture on low speed.

Add half the flour mixture and mix until just incorporated.

Repeat adding the remaining milk and flour mixtures, mixing until combined.

Scoop batter onto prepared baking sheets, spacing an inch apart.

Bake for 8-10 minutes, until the tops spring back when touched.

Remove from oven and let cool completely.

For the filling, add sliced strawberries to a bowl, sprinkle with granulated sugar, and let sit for 15 minutes.

Preheat the broiler and set a rack 6 inches from the broiler.

Once cookies are cooled, place them round-side down and top half with sliced strawberries.

Press 3 marshmallows together until flattened and place them on the other half of the cookies.

Place marshmallow-covered cookies under the broiler for 1-4 minutes, until lightly golden brown.

Watch carefully as the marshmallows can burn quickly.

Top the strawberry side of each cookie with a marshmallow cookie and gently press together.