Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Grease either a 23 cm square cake tin, a 25cm round tin or a 2lb loaf tin and line with baking parchment.
Stir together the grated courgette, sugar, oil, milk, vinegar and vanilla in a large bowl.
Sift together the flour, cocoa powder, baking powder, bicarbonate of soda and salt.
Add the dry ingredients to the wet and stir until no dry lumps remain. Gently fold through the chocolate chips.
Transfer the batter to the prepared tin and spread it level.
Bake for about 30 minutes for a square cake, or 80 minutes for a loaf. The cake is ready when it is firm to the touch and a skewer inserted into the centre comes out clean, or with a few moist crumbs but no wet batter.
Leave the cake to cool in the tin for 15 minutes then carefully turn it out onto a wire rack and leave to cool completely.
