Melt the white chocolate in a bowl over a pan of barely simmering water. Beat together the butter and icing sugar, then beat in the white chocolate. Set aside.
Line the bottom of a 20cm springform tin with parchment paper. Working carefully, spread a thin layer of white chocolate buttercream on a biscuit and place around the edge of the tin. Repeat this until all biscuits are used, creating a spiral of cookies and frosting. Top the cookies with a layer of white chocolate buttercream and place in the freezer for up to two hours to set the buttercream. Remove from the freezer and remove the springform pan. Cover the cake completely with the remaining white chocolate frosting. Set the cake in the fridge for 30 minutes to set.
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 incorporate steadily more cream to 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.
Let to one side for 20 - 30 minutes until thickened but still pourable.
First, ensure you have a cold cake. Also make sure the sides and tops are completely smooth. Any dips, bumps, or imperfections will show in the final result.
Place cake over a cooling rack and pour ganache to cover the cake. Allow ganache to set for 30 minutes and place on a serving platter. Pipe a border of the remaining ganache around the top and bottom of the cake. Top with chocolate shavings and serve. Cake will keep up to 3 days refrigerated.