Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.

In a microwave-safe bowl, melt the semi-sweet chocolate and butter together, stirring until smooth. Let cool slightly.

In a medium bowl, whisk together flour, cocoa powder, baking powder, and salt.

In a large bowl, beat brown sugar and eggs until smooth. Stir in vanilla extract and the melted chocolate mixture.

Gradually add the dry ingredients to the wet mixture, folding gently to combine.

Drop tablespoon-sized scoops of dough onto the prepared baking sheets, spacing them about 2 inches apart.

Bake for 10-12 minutes until set but still soft.

Remove from the oven and immediately press a halved marshmallow into the center of each cookie.

Return to the oven for an additional 2 minutes to soften marshmallows.

Remove cookies to wire racks to cool while preparing the icing.

For the icing, whisk powdered sugar, melted butter, cocoa powder, hot water, and vanilla extract until smooth and glossy.

Spread or drizzle icing over cooled cookies and decorate with assorted sprinkles.

Allow icing to set before serving.