Break the digestive biscuits into bite size pieces and place in a bowl. Fold in the nuts.
In a saucepan, add the sugar, cocoa powder and milk.
Cook on medium heat for a few minutes until the sugar and cocoa powder are well incorporated.
Add the butter and cook for a few more minutes until the butter has melted completed and the chocolate syrup is gently simmering.
Pour the chocolate syrup into the digestive biscuit pieces and mix well.
Transfer to a round baking pan and press into the base. Place in the fridge for about 1 hour.
Meanwhile, make the chocolate ganache.
Gently heat the heavy cream and add the chocolate. Keeping the heat very low cook until the chocolate is completely melted. Alternatively, you can use a microwave for this step, and heat the chocolate and whipping cream together at 5 second intervals, until the chocolate is melted and your ganache is ready.
Remove the cake from the fridge, pour over the chocolate ganache, and sprinkle with more slivered pistachios and sliced almonds.
Transfer to the fridge for at least 4 hours or overnight.
Slice the cake and serve immediately.