Grease and line a 23 cm springform tin with baking paper and set aside.
To make the base, process the chocolate biscuits in a food processor until fine crumbs form. Transfer to a large mixing bowl.
Place the butter, condensed milk, chocolate melts and 2 tsp Easter Hot Chocolate in a saucepan over low heat. Warm gently, stirring until melted and smooth.
Pour the butter mixture over the biscuit crumbs. Add the hazelnuts and rice bubbles and mix until evenly coated. Press firmly into the base of the prepared tin (using your fingers or the flat base of a glass) to form an even layer. Refrigerate while preparing the mousse.
For the mousse, place the condensed milk, 100 mL thickened cream, dark cooking chocolate, chocolate melts and 3 tsp Easter Hot Chocolate in a saucepan over low heat. Stir gently until melted and well combined. Transfer to a large bowl and allow to cool slightly.
Whip the remaining 500 mL thickened cream until stiff peaks form. Gradually fold the whipped cream into the cooled chocolate mixture in batches until smooth and evenly combined.
Pour the mousse mixture over the chilled base, smoothing the top. Refrigerate for a minimum of 4 hours, before adding the ganache.
To make the ganache, heat the cream in a saucepan until just hot. Place the chocolate in a heatproof bowl, pour over the hot cream, add the corn syrup and whisk until smooth and glossy.
Once the mousse has set for at least 4 hours, pour the ganache over the cake and return to the refrigerator until it is set (minimum 8 hours or overnight).
To serve, remove the cake from the tin, transfer to a serving platter. Dust with Easter Hot Chocolate and decorate with crushed hazelnuts.
