Preheat the oven to 350 degrees F. Line an 8 inch square pan with foil and grease foil or spray with cooking spray.
In a microwave safe mixing bowl, heat the butter and chocolate on high for 30 seconds. Stir well, then heat on high, stirring at 20 second intervals, until mixture is melted and smooth.
With a heavy duty scraper (or a handheld mixer), add the marshmallow cream and stir until well mixed. Stir in sugar until well mixed. Add the eggs and stir until blended, then stir in the vanilla.
Sift (or just stir very thoroughly) the flour, baking powder and salt, then stir into the batter. Spread in the pan and sprinkle top with chocolate chips.
Bake at 350 for about 30 minutes. Internal temperature should be around 210 degrees F and they will still seem very fudgy. Let cool completely.