Melt half a cup of unsalted butter and twelve ounces of semisweet chocolate in a medium saucepan over low heat, stirring frequently until silky and melted. Remove from heat and allow to cool slightly.

In a medium bowl, whisk together one and a half cups of flour, a quarter cup of unsweetened cocoa powder, one and a half teaspoons of baking powder, and a quarter teaspoon of salt.

In an electric mixer, beat one and a quarter cups of brown sugar together with three eggs and one and a half teaspoons of vanilla extract until just combined. Slowly pour in the cooled chocolate mixture and mix until smooth.

Gradually add the flour mixture, mixing until just combined. Cover the dough and chill in the refrigerator for at least one hour.

Preheat the oven to 325°F (163°C). Line two baking sheets with parchment paper.

Scoop tablespoon-sized balls of dough, roll them into smooth spheres, and flatten slightly on the prepared baking sheets, spacing them about two inches apart.

Bake for twelve minutes. Then, press halved marshmallows into the center of each cookie and return to the oven for an additional two to three minutes, until the marshmallows are soft and bubbly.

Let cookies cool briefly on the baking sheets before transferring to a wire rack to cool completely.

Melt four tablespoons of unsalted butter in a small bowl, then whisk in two cups of powdered sugar, a quarter cup of cocoa powder, and a quarter cup of hot water until smooth and glossy.

Stir in half a teaspoon of vanilla extract.