To make the base, put the biscuits in a food processor and whizz for 2 minutes until smooth and sandy. Transfer to a mixing bowl and stir in the melted butter.
Tip the mixture into a 22cm (8½in) round springform tin and press into the base using the back of large spoon, until even and smooth. Cover and chill while you make the filling.
Put the milk and dark chocolate in a heatproof bowl set over a pan of simmering water. Leave to melt, stirring occasionally, until smooth and glossy. Remove from the heat and set aside to cool a little.
Meanwhile, using a free-standing mixer or electric hand whisk, beat the mascarpone and soft cheese with the vanilla extract and double cream until smooth, then gradually pour in the melted chocolate, mixing until combined. Pour the mixture into the tin and smooth the top. Cover and chill for 4 hours or overnight.
For the almond brittle, line a baking tray with nonstick baking paper and scatter over the almonds. Set aside.
Put the sugar in a nonstick pan and gently heat, without stirring, until the sugar has completely dissolved. When it turns a light caramel colour, remove from the heat and carefully pour it over the nuts in a thin layer. Leave to set for 5-10 minutes, then break into medium-sized shards.
Transfer the cake to a serving plate and arrange the brittle on top. Serve with a drizzle of the warm salted caramel sauce, if you like.