Preheat the oven to 350°F (175°C). Grease an 8-inch (20cm) loaf pan and line it with parchment paper, leaving an overhang on the sides for easy removal
In a large bowl, combine the milk and lemon juice. Let the mixture sit for 5–10 minutes until slightly curdled. Add the sugar and vanilla extract to the milk mixture and whisk until the sugar is well combined.
Add the flour, baking powder, baking soda, and salt. Whisk until just combined and smooth. Add the melted butter then whisk until fully incorporated. Do not over mix.
Transfer about one-third of the batter to a separate bowl. In a small bowl, mix the cocoa powder and water until smooth, then stir it into the reserved batter to create the chocolate batter.
Spoon the vanilla and chocolate batters into the prepared loaf pan, alternating between the two to create layers. Aim for about two layers of each batter. Gently tap the pan on the counter to level the batter, then lightly swirl with a knife for a marbled effect.
Bake for 45–50 minutes, or until the top is set and a toothpick inserted into the center comes out clean. The loaf may have a slight crust when freshly baked, which will soften as it cools.
Let the cake cool in the pan for 30 minutes. Using the parchment overhang, carefully lift the loaf out of the pan and transfer it to a wire rack to cool completely.