Preheat the oven to 170°C. Grease and line the base of a round cake tin.
Put about 100g of walnuts into a small food processor and blend until you get a coarse mixture. Measure out a cupful.
Grate the carrot into a large bowl. Add the sugar, walnut and sultanas, then combine. Add the flour, baking powder, oats and cinnamon.
Whisk the eggs in a small bowl. Add the oil and vanilla, then combine. Pour this into the batter mixture and mix gently, until well combined.
Tip the batter into the cake pan.
Bake in the middle of the oven for 35-40 minutes, or until an inserted skewer comes out clean.
Leave the cake in the pan for 10 minutes to cool, then turn it out onto cooling rack to cool completely.
For the icing: beat some cream cheese and icing until smooth and fluffy. Spread this over the cake then scatter some chopped walnuts on top (or any remaining walnut crumble).