Pre-heat oven to 160c fan forced
Sift and separate flour into two containers:
First container with 125g self-rising flour.
Second container with 100g self-rising flour and 25g of cocoa powder
Line a 22cm springform tin with silicon paper, base and side.
In a mixer with a whisk attached, whisk eggs with sugar and vanilla on medium to high speed for 8 to 9 minutes, or until eggs have doubled in volume, still whisking drizzle in the oil, little at the time, until all the oil has been fully incorporated, now separate mixture into two equal bowls.
Into the first bowl: gently fold in 125g of self-rising flour.
Into the second bowl: gently fold in 100g of self-rising flour
with the 25g of cocoa powder.
Without overmixing, mix in to combine 60g of milk into each bowl.
Dollop a few spoonfuls of batter at the time alternating between the chocolate and vanilla batter into the prepared cake tin ,until all the batter has been used up.
Sprinkle the extra sugar over the batter, then bake in a pre-heated oven for about 30 to 35 minutes, use a skewer to check for doneness.
When cooked, remove cake from the oven, rest for 30 to 40 minutes before removing cake from baking tin.