Preheat the oven to 375°F. Gently melt the chocolate over a double boiler or in the microwave. Set aside to cool. Using an electric mixer, cream the softened butter and brown sugar together until smooth. Mix in one egg at a time. Add the vanilla and cooled melted chocolate. Mix to combine, scraping down the sides of the bowl as needed. Sift the flour, cocoa powder, baking soda, and salt together in a separate bowl. Slowly mix in half of the flour mixture on low speed. Scrape down the sides of the bowl. Mix in the rest of the flour mixture. Increase the speed to fully incorporate all of the ingredients. Once the dough comes together, refrigerate it for about 20 minutes so it’s easier to work with. Use a spoon to portion out the cookie dough onto a baking sheet, about 1-2 tablespoons per cookie. Bake the cookies for 10-12 minutes. Transfer the cookies to a cooling rack.
Combine the marshmallows and water in a double boiler. Slowly melt, stirring often so that the marshmallows do not burn. After a few minutes, the marshmallows should have melted down to a silky smooth glaze. You can adjust the consistency with more or less water. Work quickly to coat half of each cookie with the glaze. Immediately sprinkle the toppings onto each cookie. Let cool before serving.
If the marshmallows clump up and become too thick to work with while coating the cookies, return the bowl back to the stovetop over the simmering water. Add a touch more water and let the marshmallows re-melt before continuing to glaze.