Chocolate Marshmallow Surprise Cookies

For the cookies: 

• 3 1/2 c. all-purpose flour
• 1 1/2 c. unsweetened cocoa powder (not Dutch-process)
• 1 t. baking soda
• 1 t. salt
• 1 c. unsalted butter, room temperature
• 2 c. granulated sugar
• 2 large eggs
• 1 c. whole milk
• 2 t. pure vanilla extract
• About 30 marshmallows cut in half crosswise. 

For the frosting:

• 6 c. confectioners' sugar
• 3/4 c. unsalted butter, room temperature
• 1/2 c. plus 1 T. unsweetened cocoa powder
• 3/4 c. whole milk
• 1 1/2 t. pure vanilla extract

Directions:

1. Preheat oven to 350 F. Sift together flour, cocoa, baking soda, and salt into a bowl

2. Mix butter and sugar on medium-high speed until pale and fluffy--about two minutes. Reduce speed to medium-low; mix in egg, milk, and vanilla. Mix in flour mixture slowly until combined. 

3. Using a 1 inch cookie scoop, drop dough onto cookie sheets lined with parchment paper, spacing 2 inches apart. Bake cookies, rotating half-way though, until firm, about 8-9 min. Immediately press a marshmallow half on top of each cookie.  Bake until marshmallows begin to melt, about 1 min. Let cool completely on wire racks.

4. Make frosting: Put confectioners' sugar in a medium bowl. Melt butter with cocoa powder in a saucepan over medium-low heat, stirring occasionally. Add butter mixtures to confectioners' sugar. Whisk in milk and vanilla. 

5. Spread frosting on top of each cookie enough to cover the marshmallow. Let stand until set, about 10 minutes.