1. Preheat the oven to 180°C / 160°C Fan / Gas 4 / 350°F

2. Butter a non-stick rectangular baking tin approximately 24 x 17 x 4 cm. If not non-stick, line the base with baking paper.

3. Break the 75 grams of chocolate into pieces and put in a microwave-safe bowl.

   Melt on full power at 10 - 30 second intervals, stirring between each one.

   Alternatively, melt the chocolate in a bowl above a pan of gently simmering water: stir constantly and make sure the water doesn't touch the bottom of the bowl.

   When the melted chocolate's completely smooth, set aside to cool.

4. Sift together the flour, baking powder and salt then set aside.

5. In a large bowl and preferably using electric beaters, beat together the butter, sugar and vanilla extract until soft and light. Scrape down the sides of the bowl as necessary during this and subsequent steps.

   Beat in the eggs one at a time along with approximately 1 tbsp of the sifted flour mixture each time.

   Beat in the milk a tablespoon at a time with another scant tablespoon of the flour mixture each time.

   Fold in the remaining flour.

6. Divide the cake batter in half (use scales for accuracy) and put each half in a separate, clean bowl.

   Fold the orange zest into one half and the melted chocolate into the other.

7. Spoon HALF of the chocolate batter into the base of the prepared tin: it doesn't need to completely over the base, just spread it out a little.

   Spoon over HALF of the orange batter. Avoid spreading it with a spoon, palette knife or similar as this could squash the layers together and the cake won't be marbled. Instead, tilt the tin to get it to spread then tap it firmly down on the worksurface a few times to knock out any air gaps.

   Spoon over the remaining chocolate batter, tilting and tapping as before to spread it.

   Spoon over the remaining orange batter then tilt and tap so that cake batter covers the base of the tin.

   Take a knife or skewer and swirl it through the mixture to create a marbled effect.

8. Put in the oven and bake until risen and golden brown: when done the edges should be pulling away from the sides of the tin, the top springy when lightly pressed, and a skewer or cocktail stick should come out clean (approximately 25 min).

   Tip: check after 15 minutes and if the bake is browning too quickly before it's set, turn the temperature down a little.

9. When done, transfer the baking tray to a wire rack and leave to cool for 5-10 minutes.

   Carefully remove the cake and put directly on the wire rack until completely cold.

Chocolate topping

1. Break the 100 grams of chocolate into pieces and put in a microwave-safe bowl with the butter plus the orange extract if using or extra zest if liked.

   Melt on full power at 10 - 30 second intervals, stirring between each one until completely smooth.

   Alternatively, melt in a bowl above a pan of gently simmering water: stir constantly and make sure the water doesn't touch the bottom of the bowl.

2. When the cake is completely cold, pour and spread the topping all over it.

   Sprinkle with orange zest and leave to set. Cut into squares to serve.

   Store in an airtight container and eat within 4 days.