Pre-heat the oven to 160C Fan/180C/350F/Gas Mark 4 and grease and line a 2lb loaf tin
In a large mixing bowl, stir together the self raising flour, wholemeal flour, baking powder, bicarbonate of soda, cocoa powder and coconut sugar until well combined
Grate the courgette and squeeze it over the sink to release any extra liquid, then add it to the bowl along with the dark chocolate chips, stir everything together
In another bowl or large jug, whisk together the eggs, vegetable oil, vanilla extract and yoghurt
Pour the egg mixture into the bowl of flour/courgette etc and stir everything together until well combined
Pour the mixture into the loaf tin and bake for 1 hour and 10 minutes, or until a skewer inserted in the centre comes out clean (I would suggest checking it after 1 hour). Leave to cool fully
To decorate, melt the dark chocolate in the microwave. To do this, blast it for 30 seconds, stir, then blast in 10 second bursts stirring in between each one until melted. Drizzle it over the cake
Store leftovers in an airtight continer in a cool place and eat within 3 days