PREPARATION:

1. Preheat the oven to 330°f / 165°c. Grease and line a 1 lb (8.5" x 4.5" x 2.5" (21.5 x 11.5 x 6.3cm) loaf pan with two pieces of parchment paper so that the entire of the inside of the pan is lined. Leave the edges overhanging to form a sling to allow for easy removal. Clip the edges down with binder clips if needed. 

CHOCOLATE PORTION:

1. Melt the chocolate in 30 second intervals in the microwave, stirring between each interval if needed. Alternatively melt in a heatproof bowl over a pot of simmering water, making sure that the water does not touch the bowl. Leave the chocolate to cool while you make the rest of the cake, then fold in the 30g measure of sour cream once it has cooled. 

VANILLA PORTION:

1. In the bowl of a stand mixer fitted with the paddle attachment or in a large bowl using an electric mixer, cream together the butter, sugar, and vanilla until light and fluffy, and the mixture has lightened in colour. Scrape down the sides once or twice to ensure that all the mixture is evenly creamed. 
2. Add in the eggs one at a time, mixing well after each addition and scraping down the sides as needed. 
3. Add in the sour cream, and mix to combine. In a medium mixing bowl, combine the flour, salt, and baking soda. Add to the mixture and mix until just incorporated. Scrape the bowl to ensure there are no unmixed pockets of butter at the bottom of the bowl. 

ASSEMBLY 

1. Remove 250g of the vanilla cake batter from the mixing bowl and transfer to a medium bowl. If you haven't already, mix the 30g sour cream measure into the cooled chocolate and mix to combine. Add the cooled chocolate and sour cream mixture and mix until combined and homogenous - I use the electric hand mixer for this. 
2. Using a 1 tbsp scoop for the chocolate batter and a 2 tbsp scoop for the vanilla, or just eyeballing it, blob the chocolate and vanilla batter into the pan. There is less chocolate batter than vanilla so make sure that the chocolate blobs are smaller than the vanilla. I like to do it in three layers - the first layer to almost cover the bottom of the pan, then bang the pan on the counter to remove any bubbles and give the batter a swirl using a butter knife. Then, repeat the blobbing process with more chocolate and vanilla batter. Again, bang the pan on the surface to remove any big bubbles and swirl the batter. Do this one more time with the remaining batter.
3. Swirl the mixture well using a butter knife, going horizontally and vertically across the pan. Swirl as much or as little as you like. Give the pan one final bang on the counter to make sure there are not any bubbles. 
4. If using, pipe a small line of butter down the middle of the batter to help give an even split in the cake. 
5. Bake the marble cake for 1 hour 15 minutes, checking after about an hour. The outside should be a deep golden brown and a skewer inserted into the cake should come out clean or with a few moist crumbs attached. 
6. Leave the cake to cool in the pan for 15-20 minutes then use the parchment paper as a sling to remove from the pan and transfer to to a wire rack to cool completely. 
7. Store the cake in an airtight container at room temperature for up to a week.