Preheat the oven to 160 C / Gas 2/3. Grease and line a 23cm deep round cake tin with baking parchment.
For the cake:
In a double boiler or a bowl over a pan of boiling water, gently melt the chocolate, butter and milk together until smooth and well mixed. Remove from the heat.
Into a mixing bowl, sift the flour and cocoa powder then add the sugar. Fold the chocolate mixture into the flour mixture then add the eggs, one at a time, and beat until well mixed. Finally add the soured cream and vanilla extract.
Transfer the mixture to the prepared cake tin.
Bake in the preheated oven for 1 hour or until a skewer inserted into the centre comes out clean. Leave the cake in for another 10 minutes if the centre is not quite done. Avoid opening the door of the oven until at least 45 minutes is up, this will help to prevent the cake from sinking. Remove from the oven and allow to cool completely in the tin.
For the ganache:
Bring the cream to a boil in a saucepan over a high heat. Once boiling, remove from the heat and add the golden syrup, chocolate and butter. Stir until smooth and shiny. Allow to rest at room temperature for 30 minutes to thicken.
Remove the cake from the tin and transfer to a serving plate. Spread the ganache generously over the top and sides of the cake, using a spatula or the back of a spoon to create attractive patterns. Slice and enjoy.