Preheat the oven to 350°F (175°C). Grease an 8-inch (20 cm) loaf pan and line it with parchment paper, leaving an overhang for easy removal.
In a medium bowl, mix the milk and white vinegar or lemon juice. Set aside for 15 minutes until the mixture curdles. Add the light brown sugar, cane or castor sugar, oil, and vanilla extract, and whisk until the sugars are fully dissolved.
Whisk in the sifted cocoa powder until smooth. Add the sifted flour, baking powder, baking soda, and salt, and whisk just until combined and lump-free.
Pour the batter into the prepared pan and smooth the top. Using a knife, gently draw a line about 1/4 inch deep, down the center of the batter to help the loaf rise evenly.
Bake for 40–45 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs. Let the loaf cool completely before removing from the pan.
Storing in an airtight container or under a cake cloche will keep the loaf cake moist for many days.