Preheat the oven to 180 C / Gas 4. Grease and flour a 22x33cm baking tin.
In a large bowl, mix together, oil, sugar and vanilla until well blended. Combine flour, cocoa powder, bicarbonate of soda and salt; stir into sugar mixture. Fold in courgette, carrot and walnuts. Spread evenly in prepared tin.
Bake until brownies spring back when gently touched, 25 to 30 minutes. Allow to cool before cutting into squares.