1. Place the flour and sugar in a bowl with the yeast and mix together.
2. Heat the milk until its luke warm. Add the melted butter to the milk.
3. Make a well in the centre of the flour, add the egg yolks and a little of the milk mixture. Stir with a wooden spoon. Continue to gradually add the milk until you have a smooth batter.
4. Pour half the batter into another bowl. Add the cocoa powder to one bowl of batter and stir. You now have a plain and a chocolate batter.
5. In a clean, grease-free bowl, whisk the egg whites until soft peaks form when the whisk is removed. Divide the egg whites equally between the two batters.
6. Carefully fold in the whites until smooth and fully incorporated. Cover and leave to rest for one hour.
7. Grease a 20cm/8in round spring-form cake tin with oil and line the base with parchment paper.
8. Preheat the grill to high. Spoon some of the plain batter into the base of the cake tin and spread evenly across the bottom. Give the tin a gentle side-to-side shake to even out the top of the batter. Place on a shelf 10cm/4inch below the grill and cook for two to three minutes, or until light golden-brown.
9. Remove from the grill, add a layer of the chocolate of batter, spread out, and place under the grill for three minutes, or until dark golden-brown.
10. Continue layering and grilling until you have 10 layers alternating in colour. (Or continue until you have used all the batter.)
11. Remove from the grill and leave to cool in the tin for five minutes. Carefully release from the tin and turn out onto a wire rack to cool.
For the chocolate topping:
1. Melt the chocolate and golden syrup, in a bowl sat over a pan of simmering water. Remove from the heat, allow to cool for five minutes or until to a coating consistency.
2. Melt the white chocolate in the same way and place in a small piping bag. Place a large piece of greaseproof paper under the wire rack holding the cake. Pour the topping evenly over the cake to cover completely.
3. Use a pallet knife to spread out the chocolate and to cover the sides. Any excess glaze will be caught on the greaseproof paper and can be reused to fill in any unglazed areas of the cake.
4. Snip the end from the piping bag and pipe a pattern on the top of the cake in white chocolate. Leave to set before slicing.