Step 1

Preheat oven to 375 degrees F. Line two baking sheets with parchment paper; set aside.

Step 2

In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt; set aside.

Step 3

In the bowl of a stand mixer fitted with the paddle attachment, cream together butter and sugar until light and fluffy, about 2 minutes. Add egg, milk, and vanilla, and beat until well combined. Add reserved flour mixture; mix on low speed until combined.

Step 4

Using a tablespoon, drop dough onto prepared baking sheets, about 2 inches apart. Bake until cookies begin to spread and become firm, 10 to 12 minutes.

Step 5

Remove baking sheets from oven, and place a marshmallow, cut-side down, in the center of each cookie, pressing down slightly. Return to oven, and continue baking until marshmallows begins to melt, 2 to 2 1/2 minutes. Transfer cookies to a wire rack to cool completely before frosting.

Step 6

To make frosting, place confectioners’ sugar in a medium bowl. Whisk in butter and cocoa powder. Add milk and vanilla, and whisk until well combined.

Step 7

Spread about 1 tablespoon of frosting over each marshmallow, starting in the center and continuing outward until marshmallow is covered.