For the ice cream: to make the ice cream, whisk the double cream, condensed milk and vanilla in a large bowl until very soft peaks start to form - the mixture still wants to be pourable.
Divide the mixture in half and add the melted chocolate and cocoa powder to one of the bowls and fold through.
Keep the cookies in their sleeve and pour the ice cream over them, alternating between vanilla and chocolate to create a marbled effect.
Place in the freezer for at least 4 hours.
Make the ganache: for the ganache, put the cream and golden syrup into a medium saucepan and bring to a boil. Put the chocolate in a heatproof bowl and as soon as the cream is boiling, pour over the chocolate. Leave for 1 minute. Using a whisk, mix from the middle of the bowl, in small motions, to steadily incorporate more cream into the chocolate. If you don’t whisk like this, the mixture may split. Once all the cream has been incorporated, add the vanilla and butter.
Leave to one side for 20 - 30 minutes until thickened but still pourable then pour over the ice cream cakes.
Decorate with the chocolate shavings, then cut into slices to serve.