Line your 8"x12" tray bake tin with parchment paper and set it aside. Preheat your oven to 170C/340F while you make your carrot cake batter.
In a large jug or bowl, tip together your oil, buttermilk, eggs, vanilla and sugars. Whisk to combine, then add in the grated carrots and stir to mix it all together. Set aside.
In a separate large bowl, sift together your flour, baking powder, bicarbonate of soda (baking soda), salt and spices, giving it a quick mix together. Pour in your wet ingredients and mix it all together with a spatula. Fold in the broken walnuts.
Tip your carrot cake mixture into your prepared tin and spread into the corners, levelling the top. Bake in your preheated oven for 30-35 minutes until springy to touch and a wooden skewer inserted comes out clean. Cool for five minutes in the tin, then transfer to a cooling rack and cool completely.