Adjust oven rack to middle position. Preheat oven to 350°F.
Line a 8x8 inch pan with parchment paper, leaving a small overhang on all sides so that you can easily remove the brownies. Spray the parchment with nonstick cooking spray.
In a large bowl whisk the boiling water and cocoa powder until smooth. Add the dark chocolate and whisk until the chocolate is melted. Next, whisk in the butter and oil. (Mixture might look clumpy and curdled, that's ok).
Whisk in the egg, egg yolk, and vanilla, stirring until smooth.
Whisk in the sugar until thoroughly combined. Using a spatula fold in the flour, salt, chocolate chips, and mini marshmallows. Do not overmix.
Transfer the batter to the prepared pan. Spread the batter into an even layer in the pan.
Place the marshmallow cream in a microwave safe bowl. Heat for 10-15 seconds. Dollop the fluff along the top of the brownie batter. Use a knife to swirl the fluff into the batter.
Bake for 28-30 minutes or until a toothpick inserted in the center comes out with a few moist crumbs. Be careful not to overbake as this makes brownies dry.
Remove pan from the oven and place on a wire rack to cool completely. (about 1 ½ hours).
When ready to serve, lift the parchment overhand and cut the brownies on a cutting board. Enjoy!