To make the cookie dough place the butter and 12 ounces of chopped semisweet chocolate in a heat proof bowl and microwave in 30 second intervals, stirring after each heating, until the chocolate is melted and smooth.  Allow to cool for 10 minutes.

In a medium bowl whisk together the flour, cocoa powder, baking powder, and salt.

In a large bowl combine the sugar, eggs, and vanilla.  Mix well. 

Add in the melted chocolate and butter mixture and mix until blended.

Add the flour to the chocolate mixture and mix until just combined.

Cover and chill the dough for at least 1 hour or overnight.

Preheat the oven to 350 degrees. Line 2 baking sheets with parchment.

Scoop the dough out a tablespoon at a time and roll them into balls. Place 2-3 inches apart.

Bake for 10-12 minutes or until the tops of the cookies crack.

Meanwhile, cut the marshmallows in half and cut the remaining chocolate into small squares that will fit underneath the marshmallows.

Remove the cookies from the oven. Press each marshmallow half onto a chocolate square and push the chocolate square down into each cookie.

Place cookies back in the oven for 3 minutes or until the marshmallows soften.

Remove from oven and cool 2-3 minutes then remove to a wire rack to cool completely.