Preheat the oven to 160°C, fan 140°C, gas 3. Grease 2 x 20cm cake tins and line the bases with baking parchment.
Sift the flour and cocoa into a large bowl. Stir in the sugar and make a well in the centre. Spoon in the golden syrup.
In a separate bowl, whisk together the eggs, sunflower oil and milk. Pour into the flour mixture and mix to a smooth batter. Divide the mixture between the 2 prepared tins and bake for 30-35 minutes, until the cakes are well risen. Turn out onto a wire rack and leave to cool.
To make the chocolate icing and filling, grate 10g each of the plain and milk chocolate into a bowl and set aside. Finely chop the remaining chocolate and place in a small, heatproof bowl over a pan of simmering water (making sure it doesn't touch the water) for a few minutes until melted. Remove from the heat and stir until smooth. Slowly whisk in the soured cream, then let sit for a few minutes until thick enough to spread.
Spread half of the icing over the top of one cake, then place the other cake on top of it and spread the remaining icing over the top. Sprinkle over the grated chocolate to decorate.