Preheat the oven to 180°C (fan) and line two large, flat baking trays with greaseproof paper.
Beat the butter and two sugars together until smooth and combined.
Mix in the egg and vanilla.
In a separate bowl, stir the flour, mixed spice and salt together to combine. Tip into the butter mixture and fold everything together to form a cookie dough.
Add the grated carrot, grated orange zest and chopped walnuts. Fold into the cookie dough.
Drop large tbsps of cookie dough onto the baking trays (you should get 16 in total) leaving 8-10cm between each so they have room to spread out. I normally bake 4-5 cookies on each baking tray at a time.
Bake the cookies for 10-12 minutes until golden on the outside and pale and soft in the middle. Don't worry if the middle still looks under-baked, this is how they are meant to be and the cookies will carry on baking as they cool down.
Place the baking tray on a wire rack and immediately run a large round cookie cutter around the cookies to make them perfectly round.
Leave the cookies on the baking tray to cool completely.