For the cookies:
1 3/4 cups all-purpose flour
3/4 cup unsweetened cocoa powder
1/2 teaspoon baking soda
1/2 teaspoon coarse salt
1/2 cup (1 stick) unsalted butter, at room temperature
1 cup granulated sugar
1 large egg
1/2 cup whole milk (I had only fat free)
1 teaspoon pure vanilla extract
About 15 marshmallows, halved crosswise
For the frosting:
3 cups confectioners' sugar
6 tablespoons unsalted butter, room temperature
1/4 cup plus 1 1/2 teaspoons unsweetened cocoa powder
1/4 cup plus 2 tablespoons whole milk (I used fat free)
3/4 teaspoon pure vanilla extract
1. Preheat theoven to 375 degrees. Make 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 paddle attachment. I had to use my hand held mixer. I do not have a stand mixer but I am hoping to get one soon. Mix on medium-high speed until pale and fluffy, about 2 minutes. Reduce 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 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 milk and vanilla.
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 container at room temperature up to 2 days.