• Preheat oven to 180 °C (350°F). Line 20x20 cm (8x8 inch) baking pan with baking paper. Set aside.
• In a bowl whisk together flour, cocoa powder and salt. Set aside. Add chocolate, butter and sugars in a heatproof bowl and set over a pan of simmering water, stirring occasionally until melted and smooth, and mixture is glossy, about 4-5 minutes. We want sugars to melt a bit too.
• Allow mixture to cool slightly, then add in vanilla and eggs one at a time, whisking in between each addition.
• Fold in dry ingredients until just combined.
• Pour the brownie batter into the bottom of prepared pan and smooth into an even layer. Bake 25-30 minutes until a toothpick comes out mostly clean (there should be some moist crumbs since we want brownies to be fudgy). Allow to cool completely before spreading marshmallow mousse on top.

Marshmallow mousse

• In microwave safe bowl, add marshmallows and 80 grams (2.8 oz) of heavy cream. Heat at intervals of 30 seconds until marshmallows are completely melted, stiring after each 30-second interval. Leave to cool to lukewarm, stirring occasionaly to avoid making skin on top.
• Using an electric mixter beat the rest of the heavy cream until soft peaks form. Using a rubber spatula, fold in cooled marshmallow creme until fully incorporated, and mixture is smooth and lump free. Pour over cooled brownies and smooth into an even layer. Leave to harden.

Chocolate ganache

• Place chopped chocolate in a bowl. Heat the cream in a small saucepan until it begins to gently simmer, then pour it over the chocolate. Let it sit for 2-3 minutes to gently soften the chocolate, then stir until smooth. You should get a smooth runny ganache that's easily pourable. Pour over hardened mousse, spread with an offset spatula and leave to cool, for the ganache to harden.
• Slice and enjoy. Store in the refrigerator in an air-tight container, but serve at room temperature (Or you can eat it cold. They taste delicious both ways.)