Oil / butter and dust cocoa powder in a rectangular 8x5 inch pan or line with baking paper. Keep aside.
Preheat oven to 180xb0C.
In a large bowl, mix all dry ingredients with a whisk (flours, soda, cocoa powder, salt).
In another bowl, mix together sugar and oil, to this whisk in the water, grated carrots.
Add the dry ingredients to the wet and combine gently. If the batter is too dry, add 2 tbsp of water extra. To this add the vinegar in the final stage, it will bubble, combine well to the rest of the batter.
If adding walnuts or chocolate chips, fold in at this stage.
Scrape the batter into the prepared tray and bake for 25-30 mins until a tester comes out clean.
Wait to cool completely before cutting into pieces.
Cut into 12 pieces. Freeze or refrigerate and serve chilled.