Preheat the oven to 190°C or 375°F and prepare the tin (mine is 12cm/5in x 20cm/8in) by greasing it. Use a bit of oil or butter. Spray would do too.
Add the eggs, and the sugar into a bowl. Whisk until fluffy using a hand-electric whisker. Drizzle in the oil little by little and whisk until this is all added and incorporated.
Add the yogurt and whisk to incorporate this. Don’t over-whisk. Add the zest and the salt.
Mix the baking powder and the soda bicarbonate with the orange juice into a tiny little container like a coffee cup or something small and add to the mixture.
Swift in the flour and the turmeric powder and using a spatula mix gently the flour into the cake mix until this gets pretty much incorporated. Don’t worry if you have a lump or two, it’s more important that you are not overly mixing the batter.
Separate the cake mix (batter) by taking approximately ⅓ away into another bowl. Add the cacao powder to this and gently incorporate this.
Place the ⅔ cake mix into the prepared tin and pour the cacao mix into a middle line over the turmeric cake mix. With a chopstick or a wooden spoon handle make some swirl movements to give the cake the marble effect. Not too many as you want the marble patterns rather than mixing this and homogenizing the two batters.
Place the marble cake into the preheated oven (make sure it’s in the middle of your oven) and bake for approximately 40 minutes. Do the skewer test and if this comes out clean and the cake has slightly shrunk, it’s ready.
Take out of the oven, and allow to cool for 5 min. Take it out of the tin completely and allow it to cool on a cooling rack.
It’s ready to serve about 30-40 min later. Enjoy with a chocolate drizzle or a scoop of ice cream if you wish.
Notes
Lemon and orange zestI use the fresh ones if possible but I don’t always have ‘zestable’ fruits. I will explain.I did learn this from my mum too. It’s the way she always preserved this ever since I remember.What is a 'zestable' fruit?For zesting lemons and oranges I always buy my lemons and oranges unwaxed and organic (whenever available).I wash these thoroughly when I need or eat the fruit and zest them in the jar/glass where I always keep them. I add a layer of sugar on top of about one finger thick. If they are waxed I just juice them but I don't use the skin.I also have a cubed orange peel candied in the same way. All need to be kept in the fridge. They do last forever.