Heat oven to 350 degrees F.
In a large bowl or stand mixer, beat butter, brown sugar and eggs until light and creamy. Add flour, 1/2 cup cocoa powder, baking powder, salt and 1 Tbsp vanilla. Mix just until combined. Fold in chocolate chips. Chill dough for 30 minutes.
In a medium bowl, whisk together melted butter, 1/4 cup cocoa powder, 1 Tbsp vanilla, powdered sugar and water. Mix until smooth.
To bake: Scoop into rounds with a cookie scoop, place on a GoodCook nonstick cookie sheet. Bake for 8-10 minutes, or just until the center of cookies are set. Remove from oven and immediately press 3-4 marshmallows and 2 walnuts into the top of each cookie. Allow to cool slightly before drizzling with chocolate icing. Cool before serving.