Melt the butter in a saucepan.

Add sugar to the butter - mix with a spoon.

Add eggs and mix vigorously.

Add flour with cocoa and mix with a spoon.

Pour the batter into a baking pan (greased with butter and floured).

Bake at 180 degrees (top/bottom heat) for 20-30 minutes - until a dry skewer comes out clean.

After baking, let it cool - do not remove the cake from the pan.

Break the chocolate into pieces into a heatproof bowl, add the heavy cream and heat together in a bain-marie (pour water into the bottom of a pot, place the bowl with chocolate on top of the pot and heat over steam until melted).

Set aside the chocolate to cool.

Whip the cold heavy cream until stiff.

Dissolve the gelatin in a small amount of cold water - use just enough to cover the gelatin. Set it aside for 5 minutes to swell.

Briefly heat the gelatin (e.g., in a microwave) - do not heat for too long so it doesn't get too hot.

Add the chocolate to the whipped cream and mix with a spatula.

If you prefer sweeter creams, you can sweeten the mousse with powdered sugar - I didn't add any.

Add a tablespoon of chocolate cream to the heated gelatin and mix.

Pour the gelatin into the rest of the cream and mix thoroughly.

Spread the mousse over the brownie and smooth the surface.

Refrigerate the cake for several hours.

Before serving, you can sprinkle the brownie with a little dark cocoa powder and chopped nuts.