1stick (8 tablespoons) unsalted butter, at room temperature
1cupgranulated sugar
1large egg
2cupwhole milk
1teaspoonvanilla extract
About 15 marshmallows, halved crosswise
For the frosting:
3cupsconfectioners' sugar
6tablespoonsunsalted butter
1/4cupunsweetened cocoa powder
1/4cupplus 2 tablespoons whole milk
4teaspoonvanilla extract
Instructions
For the cookies:
Preheat oven to 375°F. Line large baking sheets with parchment paper or silicone baking mats.
In a medium bowl, sift together flour, cocoa, baking soda, and salt.
In the bowl of an electric mixer beat the butter and sugar on medium-high speed until light in color and fluffy, 2 to 3 minutes. Reduce speed to medium-low and mix in the egg, milk, and vanilla. On low speed gradually mix in the flour mixture until combined.
Using a 1 1/2-tablespoon spring-loaded cookie scoop, drop dough onto prepared baking sheets spacing 2 inches apart. Bake cookies until firm, 8 to 10 minutes. Immediately press a marshmallow half on top of each cookie. Bake until marshmallow begins to melt, 2 minutes more. Let cookies cool completely on baking sheets on wire racks.
For the frosting:
Place confectioners' sugar in a medium bowl. Melt butter with cocoa powder in a small saucepan over medium-low heat, stirring occasionally. Add butter mixture to confectioners' sugar. Whisk in milk and vanilla until thick and smooth.
To assemble:
Spread about 1 tablespoon of frosting on top of each cooled cookie to cover marshmallow. Let stand at room temperature until set, about 10 minutes. Cookies can be stores in single layers in airtight containers for up to 2 days.