Surprise Cookies

These are amazing cookies! My mom loves them, and they make a delicious addition to any meal. They're perfect for hot-chocolate lovers as some of the same ingredients used in hot chocolate go into this chocolaty dessert. The frosting hides the marshmallows, creating a wonderful surprise for anyone who is lucky enough to eat one.

Ingredients for the Cookies:
1 3/4 cups all-purpose flour
3/4 cup unsweetened cocoa powder (not Dutch-process)
1/2 teaspoon baking soda
1/2 teaspoon coarse salt
1/2 cup (1 stick) unsalted butter, room temperature
1 cup granulated sugar
1 large egg
1/2 cup whole milk
1 teaspoon pure vanilla extract
About 15 marshallows, halved crosswise

Ingredients for the Frosting:
3 cups confectioners' sugar
6 tablespoons unsalted butter, room temperature
1/4 cup plus 1 1/2 teaspoons unsweetened cocoa powder (not Dutch-process)
1/4 cup plus 2 tablespoons whole milk
3/4 teaspoon pure vanilla extract

Directions:
1. Preheat the oven to 375 degrees Fahrenheit. To make the cookies: Sift together flour, cocoa powder, baking soda, and salt into a bowl.
2. Put butter and sugar into the bowl of an electric mixer fitted with the paddle attachment. Mix on medium-high speed until pale and fluffy, about 2 minutes. Reduce the speed to medium-low; mix in egg, milk, and vanilla. Mix in flour mixture, 1/2 cup at a time, until combined.
3. Using a 1 3/4-inch ice cream scoop, drop dough onto baking sheets lined with parchment paper, spacing 2 inches apart. Bake cookies, rotating sheets halfway through, until firm, 8 to 10 minutes. Immediately press a marshmallow half on top of each cookie. Bake until marshmallows begin to melt, 2 minutes more. Let cool completely on sheets on wire racks.
4. Make frosting: Put confectioners' sugar in a medium bowl. Melt butter with the cocoa powder in a saucepan over medium-low heat, stirring occasionally. Add butter mixture to the confectioners' sugar. Whisk in the milk and vanilla.
5. Spread about 1 tablespoon of frosting on top of each cookie to cover marshmallow. Let stand until set, about 10 minutes. Cookies can be stored in single layers in airtight containers at room temperature for up to 2 days.