01 -
Combine the butter and chopped chocolate in a microwave-safe bowl. Heat in 20-second bursts, stirring after each, until smooth. Let cool for 15 minutes.
02 -
In a medium bowl, whisk the flour, cocoa powder, baking powder, and salt until evenly combined.
03 -
In a large bowl, beat the brown sugar, eggs, and vanilla for 1-2 minutes until glossy. Beat in the cooled chocolate mixture, then fold in the dry ingredients just until combined.
04 -
Cover the bowl and chill for at least 1 hour, or until the dough is firm enough to scoop. If chilled longer than 4 hours, let it stand for 20-30 minutes before scooping.
05 -
Heat the oven to 325°F (165°C). Line baking sheets with parchment. Scoop tablespoon-size mounds 2 inches apart and bake for 10 minutes, until puffed and set at the edges.
06 -
Remove the pan from the oven. Press one marshmallow half, cut side down, into the center of each cookie. Return to the oven for 2-3 minutes, until the marshmallow is soft.
07 -
Cool cookies on the pan for 5 minutes, then move to a rack. Whisk powdered sugar, cocoa powder, melted butter, 2 tablespoons hot water, vanilla, and salt until smooth. Add more hot water a few drops at a time if needed.
08 -
Spoon glaze over each marshmallow-topped cookie. Add sprinkles if using and let set for 30-60 minutes before storing or serving.