Preheat and prepare the pan. Preheat the oven to 350 °F (≈175 °C). Line a 9×9-inch square baking pan with foil, leaving an overhang on the sides for easy removal. Liberally spray the foil with cooking spray; set pan aside.

Make the brownie batter. In a large bowl, combine the melted butter and sugar until smooth. Add the eggs one at a time, stirring well after each addition, then stir in the vanilla extract. Next, add the cocoa powder, salt, and flour, stirring until the batter comes together into a soft mixture.

Bake the brownies. Pour the batter into the prepared pan and smooth into an even layer. Bake for about 22–25 minutes, or until a toothpick inserted near the center comes out clean or with moist (not wet) crumbs. Once done, let the brownie layer cool completely in the pan.

Make the mousse layer. In a large saucepan over low heat, combine the miniature marshmallows, chopped semi-sweet chocolate, and milk. Stir frequently until everything is melted and smooth. Remove from heat and allow this chocolate-marshmallow mixture to cool completely (about 15–20 minutes) so it won’t deflate the whipped cream later.

Whip the cream. Meanwhile, in a stand mixer (or using hand beaters), whip the heavy whipping cream until stiff peaks form. This means the cream holds its shape when you lift out the whisk.

Fold mousse and assemble. Once the chocolate mixture is cooled, gently fold it into the whipped cream—start by mixing in a small portion to lighten the chocolate mixture, then fold in the rest until fully combined and still airy. Spread the mousse evenly over the cooled brownie layer in the pan.

Chill to set. Refrigerate the assembled brownies for at least 1 hour (longer is fine) so that the mousse layer firms up.

Add ganache (optional). If you’d like a glossy finish, make the ganache: Combine the chocolate chips and heavy cream in a microwave-safe bowl. Heat on high for 15–20 seconds, stir, then continue heating in 10-second increments until the mixture is smooth. Drizzle this over the set mousse layer just before serving.

Serve. Use the foil overhang to lift the dessert out of the pan, slice into squares, and serve chilled or at room temperature (your preference).