Preheat the oven to gas 4, 180°C, fan 160°C and lightly grease with butter butter and line with baking paper 2 x 20cm loose-bottomed cake tins.
For the cake, sift the flour, cocoa powder, bicarbonate of soda and the sugars into a large bowl and mix well.
Pour the sunflower oil and milk into a measuring jug. Add the vanilla extract, golden syrup and eggs, and mix together with a fork until well combined. Make a well in the dry ingredients, then whisk in the wet ingredients with an electric hand whisk until a smooth batter forms.
Divide the mixture between the lined tins, put on a baking sheet and bake for 20-25 mins or until a skewer inserted comes out clean, the sponge is slightly springy to touch and the edges of the cake are just coming away from the sides of the tin.
Remove the cakes from the oven and leave to cool in the tins for 5 mins, then turn out onto a wire rack to cool completely.
To make the white chocolate icing, melt the white chocolate in a small bowl set over a pan of gently simmering water. Stir until melted, then set aside to cool slightly. Meanwhile, in a separate bowl, beat together the butter and icing sugar with an electric hand whisk until fluffy. Pour in the melted chocolate and whisk again until smooth.
Once the cakes have cooled, place one cake bottom side up on a large serving platter, top with the white chocolate icing and then place the other cake on top.
To make the chocolate fudge icing, pour the double cream into a small saucepan over a medium heat and bring just to the boil. Put the chopped chocolate, cubed butter and golden syrup into a large bowl, pour over the hot cream and beat together with a whisk until smooth. Sift over the icing sugar and beat again until smooth. Set aside for 5 mins to cool and thicken slightly. Spread the chocolate fudge icing over the top and sides of the cake. Smooth the edges using a palette knife.
To make the white chocolate curls, melt the white chocolate in a small bowl set over a pan of gently simmering water. Once melted, pout onto a plastic chopping board and spread until it forms an even layer about 5mm. Place in fridge and chill for about 15 mins or until it is just set but not hard. Run a cheese slicer or vegetable peeler over the surface of the chocolate to make thick curls of chocolate. Make as many as you like and use to adorn the top of the cake.