Butter and line a 23cm springform cake tin. Blitz the biscuits in a food processor to form fine crumbs, then add the butter and blitz again. Tip the biscuit mixture into the tin and press down firmly with the back of a spoon. Chill in the fridge while you make the filling.
In separate bowls, melt the milk and dark chocolate over pans of gently simmering water. Alternatively, melt in the microwave in 30-second bursts, stirring between each interval until melted. Set aside to cool.
Use a hand whisk to mix the mascarpone, cream cheese, icing sugar, malted milk drink powder, hot chocolate powder and vanilla extract until smooth. In a separate bowl, whisk the cream, then fold into the mascarpone mixture. Add half the mixture to each of the chocolate bowls, stirring until well combined. Stir half the chopped Maltesers, whole Maltesers and buttons into the milk chocolate mixture; stir the rest into the dark chocolate mixture.
Spoon the dark chocolate mixture onto the biscuit base. Top with the milk chocolate mixture and smooth over the top. Chill overnight. Decorate with Maltesers before serving.