Preheat the oven to 350°F. Lightly spray a 9x13 inch pan and set aside.
In a microwave safe bowl, combine 4.25 oz semi sweet chocolate and 1/3 cup butter. Microwave in 30 second intervals, mixing until smooth. Set aside for 5 minutes to cool.
In a medium bowl, beat together the egg, sour cream, sugar and vanilla and beat well.
Add the melted chocolate into the egg mixture, and mix until fully combined. Add in the flour and baking powder, mixing until fully combined. Fold in the chocolate chips.
Pour into the prepared baking pan, and bake for 10-15 minutes, or until a toothpick inserted in the centre comes out clean.
Sprinkle evenly with marshmallows and return to the oven for 5-8 minutes, until the marshmallows puff and melt slightly. Remove from the oven.
Meanwhile, combine the chocolate chips and the coconut oil. Melt in 30 second intervals, mixing until smooth. Allow to cool for 3 minutes.
Spread the mixture over the marshmallow layer. Allow to cool for 10-15 minutes, then place in the fridge to cool completely. Cut into squares when cooled.