Preheat the oven to 180°C (350°F) convection. Grease and line a 2lb loaf tin with a sling of baking paper and set aside.
Place the butter into a medium pot and set over a medium-low heat. Allow to melt completely then remove from the heat.
110 g (1/2 cup) vegan block butter or unsalted butter
As the butter is melting, place the flour, cornflour and salt in a medium bowl and stir together to combine.
185 g (1 1/2 cups) plain white flour, 20 g (2 tbsp) cornflour, see notes for substitutes, 1/2 tsp fine table salt
Pour the hot melted butter into the flour mixture and stir together until you get a sandy, slightly bobbly mixture. Add the sugar, baking powder and bicarbonate of soda and mix together well. I like to use my fingertips here to rub all the ingredients together to ensure they're all mixed and to try to break up any large lumps.
220 g (1 cup) granulated sugar, 1 tsp baking powder, 1/4 tsp bicarbonate of soda
Take the pot (which should still be hot) you were using for melting the butter and place the chocolate into it, off the heat. Set aside and allow the residual heat of the pot to melt the chocolate. If it hasn't fully melted after ~5 minutes, place over a low heat, stir until fully melted then remove from the heat. Set aside.
65 g (2.25 ounces) dark chocolate
Place the milk, oil, vanilla, tofu/eggs and vinegar into a jug or small bowl and whisk together - using a fork is fine (or use a hand blender to whizz until smooth).
165 g (2/3 cup) non-dairy milk or cow's milk, 30 g (2 tbsp) vegetable oil or neutral oil, 2 tsp vanilla extract, 100 g blended silken tofu OR 2 medium UK eggs, 1 tsp vinegar or lemon juice
Pour the liquid little by little into the flour mixture, stirring together between additions (using a whisk or large fork here helps) until all the liquid has been added and the batter is mostly smooth (there may be a few lumps here & there which is fine). It's okay to mix this batter a bit more than a standard cake batter so don't be afraid to give it a good mix with the whisk.
Pour 300g (1 cup) of the batter into the pot of melted chocolate (OFF THE HEAT). Add the coffee (or water) and stir together - this is your chocolate batter! The batter which is left in the bowl is your vanilla batter.
65 g (2.25 ounces) dark chocolate, 2 tbsp brewed coffee or water
Layer the chocolate batter and vanilla batter into the lined loaf tin, alternating between the two flavours (I like to do this in 6 layers i.e. 3 layers of each flavour). You can also use a butter knife to gently swirl the batter together a few times - don't go overboard here or it'll just mix the batters rather than marbling them.
Bake the cake for 50-60 minutes, covering with foil in the final 10 minutes if the cake is looking too brown. A toothpick inserted into the centre of the cake should come out clean.
Allow the cake to cool in the tin for 10 minutes before tipping out onto a wire rack to cool completely. Once cooled, slice and enjoy!