Crush biscuits finely in a food processor or with a rolling pin

Mix with melted butter and cocoa powder until well combined

Press firmly into the bottom of a 9-inch springform pan

Chill while preparing the mousse

Melt chocolate in a heatproof bowl over simmering water or in microwave, then let cool slightly

In a large bowl, whip cream until soft peaks form, then refrigerate

In another bowl, whisk egg yolks with sugar until pale and thickened

Gradually whisk melted chocolate into egg yolk mixture

Fold in vanilla extract and salt

In a clean bowl, whisk egg whites until stiff peaks form

Fold whipped cream into chocolate mixture, then gently fold in egg whites

Pour mousse over prepared base and smooth the top

Cover and refrigerate for at least 6 hours, preferably overnight

Before serving, whip remaining cream and pipe around edges or spread over top

Decorate with chocolate shavings and dust with cocoa powder

Run a knife around edges before releasing springform pan