Heat the over to 350F. Line a 9x9 inch baking dish with parchment paper and spray it with nonstick spray.
Pour an inch or so of water into a small saucepan and bring it to a simmer. Put the butter in a heatproof bowl that’ll sit on top of the saucepan without touching the water. Set the bowl on the saucepan, reduce the heat to low, and let the butter melt completely, stirring it occasionally. Add the chocolate chips and stir until the chocolate is completely melted. As soon as it’s fully melted, remove the pan and bowl from the heat but keep the bowl over the warm water.
In a stand mixer fitted with the paddle attachment, beat the eggs and sugar until the eggs look lighter in color and slightly frother, about 3 minutes. Reduce the speed to low, add the chocolate mixture to the eggs, then increase the speed to medium-high and mix until completely combined.
In a separate mixing bowl, lightly whisk together the flour, cocoa powder, and salt to break up any clumps. Turn off the stand mixer, add the flour mixture and mix on medium speed until the ingredients are just combined. Reduce the mixer speed to low, slowly drizzle in the heavy cream, and continue to mix until just combined, scraping down the sides as necessary. Use a large spoon to fold the markmallow fluff into the batter, mixing just enough to get most of the fluff incorporated with just a few streaks remaining.
Pour the batter into the prepared baking dish and spread it out to make an even layer. Bake until a butter knife inserted into the center comes out clean and the top is crackly but the brownies are sticky and ooey-gooey, about 30 minutes. Let them cool completely, then cut them into 1/2 inch pieces. Freeze them in sealed freezer bags until cold before using; they will keep for up to 2 months.