Preheat oven to 180ºC/350ºF.
In a large bowl whisk the sugar with the oil for 2 minutes. Add the egg and beat until the batter starts to bubble.
Add in the yogurt, milk, lemon zest and vanilla extract and combine.
In another bowl, combine the rice flour, cornstarch, baking powder and salt and slowly add to the batter, beating until completely blended.
Pour 2/3 of the batter into a 1lb loaf tin (10inch*9inch*4,5inch)
Mix in the cocoa powder to the 1/3 of the batter and slowly pour it in the middle of the 2/3 batter in the cake pan, making waves while pouring.
Bake for 40 minutes.
Remove the cake from the oven and leave it to cool in the tin for 10 minutes.
Remove the cake from the pan and serve.