Grease a 23cm spring form pan and line the base and side with baking paper. To make the base, put the dates, oats, nuts and cocoa powder into a food processor and pulse until finely ground and starting to stick together; about 2-3 minutes. Press the mixture evenly into the base of the pan, making sure to press down firmly.
To make the mousse, soak the gelatine leaves in cold water for 5 minutes, then squeeze out the excess water. Place the soaked leaves in a pan and, over a very low heat, melt slowly. Meanwhile, whisk the double cream in a large bowl with an electric hand-held mixer or stand mixer fitted with a whisk attachment until stiff peaks form. Whisk the yoghurt, frosting and gelatine together in a separate bowl, then gently fold into the cream, until fully combined. Spread the mousse evenly over the top of the base layer. Cover and chill for 2 hours.
To make the chocolate ganache, put the chocolate and cream in a small heatproof bowl over a pan of gently simmering water. Heat until the chocolate has melted, then stir to combine with the cream. Set aside to cool for 5 minutes, then pour over the top of the mousse. Sprinkle over the pistachios over the top, then chill in the fridge for 4 hours or overnight. Remove from the pan and slice to serve.