preheat oven to 180 degrees C. Line a muffin tray with paper baking cups.
In a medium bowl mix together flour, sugar, walnuts, cinnamon, nutmeg and allspice. In a large bowl combine pumpkin, carrot, egg, buttermilk and canola. Fold in the dry mixture until mixed then spoon into prepared muffin tin.
Bake in preheated oven until a knife inserted into the centre comes out clean, about 30 minutes. Allow to cool 10 minutes in the tin then move to a wire rack to cool completely before icing.
Beat together the cream cheese, butter and vanilla. Slowly stir in the icing sugar, tasting, until your desired sweetness is reached. Continue beating until light and fluffy. Pipe onto cupcakes with a piping bag or plastic bag with the corner cut off.