Preheat the oven according to the package directions for the brownie mix. Coat a 9x13 pan with cooking spray or parchment paper.
Prepare brownies according to package directions. Remove and set aside.
While brownies are still warm, place marshmallow fluff in a microwave-safe bowl or glass measuring bowl. Heat for 30 seconds on high heat, stir. If not smooth, return for another 30 seconds and then stir again.
Pour over warm brownies, spreading out evenly using an offset spatula. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment or a large mixing bowl with an electric hand mixer, beat together the melted butter, cocoa powder, heavy cream and salt until combined.
Add the powdered sugar slowly so the mixture is still pourable, but starts to have a little body and texture.
Pour over the marshmallow layer, spreading out with an offset spatula.
While the chocolate ganache layer is still tacky, sprinkle mini marshmallows and gently press to adhere. Allow them to fully cool.
Cut brownies into squares.
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Notes
It is easiest to cut marshmallows in half using kitchen shears.