Melt the butter and semi-sweet chocolate together in a medium saucepan over low heat, stirring frequently until smooth. Once melted, let the mixture cool slightly before using it in the next steps.

Mix flour, unsweetened cocoa powder, baking powder, and salt in a separate bowl. Whisk them together until well combined to ensure a uniform texture in each cookie.

Beat the brown sugar, eggs, and vanilla extract in a mixing bowl using a stand mixer or hand mixer until the mixture becomes light and fluffy, which should take about 2-3 minutes. Then, add in the cooled chocolate mixture, mixing until fully combined.

Gradually add the dry ingredients to the wet mixture, mixing until just combined. Be careful not to overmix, as this can affect the cookie texture.

Chill the dough in the refrigerator for at least 1 hour, or until it's firm enough to handle. This helps the cookies maintain their shape while baking!

Preheat your oven to 325°F (160°C) and line two baking sheets with parchment paper to prevent sticking and make cleanup a breeze.

Scoop tablespoon-sized balls of dough and roll them into smooth balls. Place them on the prepared baking sheets and slightly flatten each ball before baking.

Bake the cookies in the preheated oven for 12 minutes. They should be soft but set on the edges when they're done.

Prepare the marshmallows by cutting them in half while your cookies are baking. It's a fun task that won't take much time at all!

Press a half-marshmallow into the center of each cookie right after you take them out of the oven. Return the cookies to the oven and bake for an additional 2-3 minutes, until the marshmallows are soft and slightly puffy.

Cool the cookies on a wire rack. Allowing them to cool will help the marshmallows set a little more, making them easier to handle.

Whisk the icing ingredients together until smooth. Combine the powdered sugar, melted butter, cocoa powder, hot water, and vanilla extract in a bowl until creamy.

Spoon the icing over the marshmallow tops of the cooled cookies and use a spatula to spread it slightly for a whimsical look.

Sprinkle assorted sprinkles over the icing immediately, before it sets, to add some color and fun!

Rest the cookies for 30 minutes to allow the icing to firm up before serving, giving your sweet creations time to shine.