Preheat oven to 350 degrees F. Line a 9x9" square baking pan with foil, extending the sides of the foil over the edges of the pan. Spray the foil liberally with cooking spray; set aside.
In a large bowl, combine the melted butter and sugar until smooth. Add in the eggs, one at a time, stirring well after each addition, followed by the vanilla. Lastly, stir in the cocoa powder, salt and flour until a soft batter forms.
Pour the batter into the prepared pan and smooth into an even layer. Bake for approx. 22-25 minutes or until a toothpick inserted near the center comes out clean or with moist, not wet, crumbs. Cool completely.
To make the mousse: In a large saucepan, combine the miniature marshmallows, chopped chocolate bar, and milk over LOW heat. Stir frequently to melt the ingredients together. Once melted and smooth, cool completely (about 15-20 minutes).
After chocolate mixture has cooled, whip the heavy whipping cream in the bowl of a stand mixer until stiff peaks form. Fold in the cooled chocolate mixture to combine. Spread the mousse over the brownies in an even layer. Refrigerate for at least 1 hour to set up.
Just before serving, drizzle with chocolate ganache if desired. Store leftovers in the fridge.
*NOTE: to make chocolate ganache, combine 1/2 cup chocolate chips and 1/4 cup heavy whipping cream together in a microwave-safe bowl. Heat on HIGH for about 15-20 seconds, stir, and continue heating in 10 second increments until smooth.