1. Preheat the oven to 350°F (175°C) and grease a baking pan.
2. In a mixing bowl, combine melted butter, sugar, and eggs until smooth.
3. Add cocoa powder, flour, and a pinch of salt, then mix until fully incorporated.
4. Fold in semi-sweet chocolate chips for extra richness.
5. Pour the batter into the prepared pan and spread evenly.
6. Bake for 30-35 minutes or until a toothpick inserted in the center comes out with a few crumbs.
7. Let the brownies cool completely before adding the mousse layer.
8. Melt semi-sweet chocolate in a heatproof bowl over simmering water or using a microwave.
9. If using, mix melted chocolate with softened marshmallows or dissolved gelatin.
10. In a separate bowl, whip heavy cream until stiff peaks form.
11. Gently fold the melted chocolate mixture into the whipped cream until smooth.
12. Spread the mousse evenly over the cooled brownie layer.
13. Refrigerate for at least 4-6 hours or overnight to set properly.
14. Once set, optionally drizzle with melted chocolate or garnish with chocolate curls.
15. Slice into squares and serve chilled.
16. Store leftovers in the refrigerator for up to 2-3 days.