Melt bittersweet chocolate, cocoa powder, and butter in a medium sized pot over low heat on the stove and stir until well blended. Remove from heat.
Add salt and sugar and stir until both are dissolved in to the chocolate-butter mixture.
Add eggs, one at a time, mixing thoroughly after each.
Stir in flour, baking powder, and vanilla and mix well.
Carefully spread the brownie batter over the cooled graham cracker crust in the baking dish.
Bake at 350 F for 18 minutes. (If you are using a glass baking pan, reduce the heat to 325 F and bake slightly longer.)
Test brownies with a cake tester and if the brownies still seem too wet, bake for one or two minutes longer. The brownies should be just slightly wet in the center when they are done baking.
Remove the brownies from the oven and sprinkle mini marshmallows evenly on top of the brownies. Keep the oven at 350 F.
Return the brownies to the oven for another 4-5 minutes, or until the marshmallows are all melted.
Remove from oven and immediately sprinkle dark chocolate chips over the top. Do not return to the oven. The heat from the brownies and roasted marshmallows will melt the chocolate chips.
Once the chocolate chips are melted, you can leave them just like they are, or you can spread them evenly over the top with a spatula for a thin chocolate layer.
Allow brownies to sit for at least 30 minutes before cutting them, if serving hot, or chill if serving cold.