Preheat your oven to 180C
Beat butter, sugar, eggs, flour, baking powder, and vanilla together
Split the batter into two bowls
In the first bowl mix into the batter the milk and cocoa powder
In the second bowl mix the zest and the juice
Grease a loaf tin with tahini or butter and flour
Scoop the batter alternating between them
Swirl with a toothpick or knife Top with the chocolate chipsBake in oven for 40-50 mins, until a toothpick inserted comes out dry