Pre-heat oven to 180°C (160°C fan-forced). Grease and line a 1.5 litre loaf tin (base measurements 10cm X 20cm X 7cm) with baking paper.
Beat butter, sugar and eggs with an electric beater until pale and fluffy.
Add flour, baking powder, salt and milk to the egg mixture. Beat for a few seconds until thoroughly combined and smooth.
Divide mixture into two bowls.
To one bowl, add the dark chocolate and cocoa powder. Mix well.
To the other bowl of batter, add chopped apricot and lemon zest. Mix well.
Spoon chocolate batter into the prepared tin covering the entire base of the tin. Spoon the apricot batter on top of the chocolate batter and spread evenly throughout the tin.
Push a wooden chopstick or a wooden spoon straight down into the batter and dredge up the chocolate batter through the apricot batter bringing it up to the surface in a swirly motion. Do this a total of 3-4 times in different spots to create a marble effect through the batter.
Bake in the pre-heated oven for approximately one hour. Check if the cake is done by inserting a wooden skewer through the middle. If it doesn't come out clean, bake for another 5 minutes.
Remove from the oven when completely cooked. Cool completely in the tin. Slice and serve.
You can use chocolate chips instead of chopped chocolate and cacao powder instead of cocoa powder.