Preheat the oven to 350 F (180 C). Butter and flour a large loaf tin (I used a 12 inch tin).
Place the butter for the topping into a medium heatproof bowl and place into the oven for 3 minutes so the butter melts (or place it in a microwave safe bowl and microwave until melted). Stir in the other topping ingredients - except the chocolate chips - to make a moist crumbly mixture. Set aside.
For the cake: In a large bowl, beat together the oil, eggs, sugar, vanilla, salt and baking soda. Stir in half of the flour, then half of the milk, then the rest of the flour, then the rest of the milk. Pour 2/3 of this mixture into a jug and set aside. To the remaining 1/3 of the batter left in the bowl add the cocoa powder and stir together.
To assemble: Pour half of the vanilla batter into the prepared loaf pan. Top with half of the chocolate batter and then half of the topping mixture. Cover with the rest of the vanilla batter, then the rest of the chocolate mixture and the chocolate chips. Take a knife and gently swirl it through the batter a few times. Cover the batter with the rest of the topping mixture and bake for 45-60 minutes until a skewer inserted into the centre of the cake comes out clean.
Leave to cool in the tin for 10 minutes before running a butter knife around the edges of the cake and turning it out onto a wire rack to cool completely.