In a saucepan, melt the butter and chocolate chips, stirring frequently, over medium heat.
In another bowl, whisk together the flour, cocoa powder, baking powder and salt.
In a large bowl, beat the sugar, eggs, and vanilla using an electric mixer at low speed. Mix in the cooled chocolate mixture until blended. Add the flour mixture slowly.
Cover the dough and refrigerate for about an hour.
Line a baking sheet with parchment paper. Scoop your dough and roll between your palms to form cookie balls. Bake at 325 F for 11 minutes or until the tops of the cookies crack. (I like to slam the pan down on the counter to help it get that crackly top)
Meanwhile, cut marshmallows in half crosswise and stick on top of your cookie. Place back in the oven and bake until the marshmallows are just softened.
Remove from the oven, let cool, and enjoy!
