Surpise Cookies

The cookies:
1 3/4 cups flour
3/4 cups unsweetened cocoa powder
1/2 teaspoons baking soda
1/2 teaspoon salt
1 stick unsalted butter, room temperature
1 cup sugar
1 large egg
1/2 cup milk
1 teaspoon vanilla extract
about 15 marshmallows. halved crosswise

The frosting:
3 cups powdered sugar
6 tablespoons unsalted butter, room temp
1/4 cup plus 1 1/2 teaspoons unsweetened cocoa powder
1/4 cup plus 2 tablespoons milk
3/4 teaspoon pure vanilla

Preheat the oven to 375 degrees.
Sift flour, cocoa powder, baking soda, and salt into a bowl.

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 minute. Reduce speed to medium-low, mix in egg, milk, and vanilla. Mix in flour mixture, 1/2 cup at a time, until combined.

Using a small ice-cream scoop, drop dough onto baking sheets lined with parchment paper, spacing 2 inches apart. Bake cookies, rotating sheets halfway through, until firm, 8-10 minutes. Immediately press marshmallows half on top of each cookie. Bake until marshmallows begin to melt, about 2 minutes. Let cool completely on sheets or wire racks.

Make frosting:

Put sugar in medium bowl, Melt butter with the cocoa powder in a sauce pan over medium heat. Stirring occasionally. Add butter mixture to sugar, whisk in milk and vanilla.

Spread about 1 tablespoon of frosting on top of each cookie to cover marshmallow. Let stand to set, about 10 minutes. Cookies can be stored in single layers in airtight containers at room temperature up to 2 days.