In a medium microwave-safe bowl, melt together butter and semi sweet chocolate chips. Stir at 30 second intervals until melted. You’ll want this melted but not hot (if too hot, it will scramble the eggs when you add them in). Let cool 5 minutes.
In a large mixing bowl, beat together chocolate mixture, brown sugar, eggs and vanilla extract. Beat until smooth (about 2-3 minutes).
In a separate bowl, stir together flour, cocoa powder, baking powder and salt. Add dry ingredients to wet ingredients and beat just until mixed. Don’t over mix.
Cover mixing bowl with food safe wrap and place in fridge for one hour.
Line three baking sheets with parchment paper or silicone baking mats. You’ll want to bake just one tray at a time.
Scoop 2-tablespoon sized balls of cookie dough and place 2 inches apart on baking sheets. Slightly press down the tops of each cookie ball.
Preheat oven to 325F.
Bake one tray at a time for about 10 minutes each. Tops should be slightly set.
Remove from oven and place 3-4 chunks of dark chocolate chunks on each cookie. Press down slightly but don’t press all the way to the bottom of the cookie.
Place a half marshmallow on the top of each cookie – placing the cut side up.
Bake for an additional 3 minutes. Remove from oven and set aside for now. Repeat steps with other baking sheets. (WATCH THE MARSHMALLOWS VERY CAREFULLY - THEY WILL BURN!!!!)
When all cookies are baked and out of the oven, turn the oven up to broil. Place one cookie sheet at a time into the oven for about 20-30 seconds to brown the tops of the marshmallows. Watch these carefully as they will burn quickly.
IMPORTANT NOTE: When broiling the marshmallows DO NOT take your eyes off of them (I mean, even for a second). They will burn. They will catch fire. You just want them to slightly brown and then take them out!!Store leftovers in a sealed food storage container 3-4 days or in the refrigerator in a sealed food storage container for a week.