In a medium saucepan (or in a microwave safe bowl, using 50% power), melt the butter and chocolate, stirring frequently. Once melted, set aside to cool slightly.
In a medium bowl, whisk together the flour, cocoa powder, baking powder and salt.
In the bowl of an electric mixer, beat the sugar, eggs and vanilla on low speed until well combined.
Add the cooled chocolate mixture and blend until just combined.
While mixing, add the flour mixture slowly and blend until just combined.
Scrape down the sides of the bowl, then cover the dough and refrigerate about 1 hour. If making the dough a day ahead, let sit at room temperature for 30 minutes before shaping.
Preheat oven to 325*F and line 2 baking sheets with parchment paper or a silpat type liner. Use a tablespoon (or a tablespoon sized cookie scoop) to scoop the dough, then roll the dough in your hands to create balls. Arrange the balls about 2 inches apart on your baking sheets, then flatten slightly.
Bake cookies about 10 minutes.
When the cookies have baked, remove from oven and press 3-4 mini marshmallows into each cookie (as pictured above).
Return the cookies to the oven and bake another 2-3 minutes. Allow the pan of cookies to cool a few minutes, then transfer cookies to cooling rack.