Grease and line the base and side of a 23cm springform pan.
Place the biscuits and cocoa powder in the bowl of a food processor and process until finely crushed. Add the butter and process until well combined. Transfer mixture to the prepared pan. Use a straight-sided glass to spread and press mixture firmly over the base and side of the pan. Place in the fridge for 30 minutes to chill.
Place the milk chocolate in a heatproof bowl over a saucepan of simmering water. Stir with a metal spoon until chocolate melts. Remove from heat.
Place the boiling water in a small heatproof glass. Sprinkle over the gelatine. Stir until gelatine dissolves.
Use an electric mixer to beat the cream cheese and sugar until smooth. Add the cream and beat until smooth. Add the gelatine mixture and stir until well combined.
Add half of the cream cheese mixture to the milk chocolate and stir to combine. Pour into the prepared pan and smooth the surface with the back of a spoon. Place in the freezer for 15 minutes or until the surface is set.
Meanwhile, place the dark chocolate in a heatproof bowl over saucepan of simmering water. Stir with a metal spoon until chocolate melts. Add the other half of the cream cheese mixture to the chocolate and stir to combine. Pour over the milk chocolate mixture and smooth the surface. Place in the fridge for 3 hours, or until completely set.
To make the glaze, warm the milk and sugar in a small saucepan till they’re hot but not yet simmering. Pour the hot milk over the chocolate and stir until the chocolate is melted and the mixture is glossy. Pour the glaze over the top of the cheesecake, making sure it reaches the sides. Transfer the cheesecake back to the fridge and chill for at least 1 hour, or until the top is set. Slice and serve.