STEP 1 Heat the oven to 180C/160C fan/gas 4. Oil and line the base of two 18cm sandwich tins. Sieve the flour, cocoa powder and bicarbonate of soda into a bowl. Add the caster sugar and mix well.
STEP 2 Make a well in the centre and add the golden syrup, eggs, sunflower oil and milk. Beat well with an electric whisk until smooth.
STEP 3 Pour the mixture into the two tins and bake for 25-30 mins until risen and firm to the touch. Remove from oven, leave to cool for 10 mins before turning out onto a cooling rack.
STEP 4To make the icing, beat the unsalted butter in a bowl until soft. Gradually sieve and beat in the icing sugar and cocoa powder, then add enough of the milk to make the icing fluffy and spreadable.
STEP 5 Sandwich the two cakes together with the butter icing and cover the sides and the top of the cake with more icing.