1.Grease a 14cm x 21cm loaf pan. Line base and two long sides with baking paper, extending the paper 5cm above edges of pan.
2.Place dark chocolate in a small heatproof bowl over a small saucepan of gently simmering water (don’t allow bowl to touch water). Stir until melted.
3.Process ricotta and sugar until smooth; transfer to a medium bowl. Stir in melted chocolate and nougat.
4.Beat cream in a small bowl with an electric mixer until soft peaks form. Fold cream into chocolate mixture. Spoon mixture into pan, cover with foil and freeze overnight.
5.To make chocolate sauce, bring cream almost to the boil in a small saucepan over low-medium heat. Add milk chocolate and stir until smooth.
6.Turn mousse cake onto a serving platter and drizzle with a little of the chocolate sauce. Scatter with raspberries and nuts. Serve sliced with remaining chocolate sauce.