Chocolate Marshmallow Surprise Cookies

From marthastewart.com. Putting it here for reference and safekeeping. These look so good!

• Ready In: 25 mins
• Serves: 24
• Yields: 24 cookies

Ingredients

• For Cookies

• 1 3⁄4 cups all-purpose flour
• 3⁄4 cup cocoa powder
• 1⁄2 teaspoon baking soda
• 1⁄2 teaspoon salt
• 1⁄2 cup unsalted butter, softened
• 1  cup sugar
• 1  large egg
• 1⁄2 cup milk
• 1  teaspoon pure vanilla extract
• 12  large marshmallows, cut in half horizontally

• For Frosting

• 2  cups confectioners' sugar
• 4  tablespoons unsalted butter, melted and cooled
• 1⁄4 cup cocoa powder
• 1⁄4 cup milk
• 1⁄4 teaspoon pure vanilla extract

Directions

1. Preheat oven to 375 degrees. In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt; set aside.
2. In the bowl of an electric 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.
3. Using a tablespoon or 1 3/4-inch ice cream scoop, drop dough onto ungreased baking sheets, about 2 inches apart. Bake until cookies begin to spread and become firm, 10 to 12 minutes.
4. 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.
5. Make frosting: Place confectioners' sugar in a medium bowl. Whisk in butter and cocoa powder. Add milk and vanilla, and whisk until well combined.
6. Spread about 1 tablespoon of frosting over each marshmallow, starting in the center and continuing outward until marshmallow is covered.