Brush the bundt tin with melted butter, ensuring you get into all the nooks and crannies and that you come up the sides high enough. Once the tin has been thoroughly brushed, sprinkle in a tablespoon or two of plain (all purpose) flour and rotate the tin, tapping as you go, to get the flour to stick to the butter. Once all the buttered part of the tin has a coating of flour, turn the tin upside down and give it a couple of firm taps to release any clumps of stuck on flour. Set the tin aside and preheat the oven to 170C/340F whilst making the carrot cake batter.
1-2 tbsp unsalted butter, melted, 1-2 tbsp plain (all purpose) flour
In a large jug or bowl, tip together the oil, buttermilk, sugars, vanilla and eggs. Whisk to combine, then set aside. Separately, grate the carrots and roughly break the walnuts into small chunks.
180 ml (¾ cup) oil (I used vegetable oil), 160 ml (½ cup + 3 tbsp) buttermilk, 100 g (½ cup) caster sugar, 150 g (¾ cup) brown sugar, 1 tsp vanilla, 2 large eggs
In a separate large bowl, sift together the flour, baking powder, bicarbonate of soda (baking soda), salt and spices, giving it a quick whisk together. Pour in the wet ingredients and gently whisk it all until mostly combined.
375 g (2 ¼ cups + 1 tbsp) plain (all purpose) flour, 1.5 tsp baking powder, 1 tsp bicarbonate of soda (baking soda), 0.5 tsp salt, 3 tsp cinnamon, 0.75 tsp nutmeg, 0.5 tsp cloves, ground, 1 tsp (½ tsp if fresh) cardamom powder
Tip in the grated carrots and walnuts and fold through using a spatula.
410 g (about 5 ½ cups) carrots, grated, 130 g (1 ¼ cups) walnuts, broken into chunks
Spoon the cake mixture into the prepared bundt tin, flattening once done and levelling the top. Bake in the preheated oven for 55-60 minutes until springy to touch and a wooden skewer inserted comes out clean. Cool for 10 minutes in the tin, then turn out onto a cooling rack and cool completely.