Preheat the oven to 180°C / Fan 160°C / Gas Mark 4. Grease a 20cm deep cake tin with non-stick baking paper.
In a large mixing bowl sift together the flour and baking powder and stir in the sugar. Add the chopped nuts and grated carrots and mix lightly.
Make a well in the center, add the eggs and oil and beat until it is mixed thoroughly.
Pour the mix into the cake tin and place in the preheated oven for around 1 hour, or until golden brown and shrinking from the sides of the tin.
Tip: Using a skewer, insert it in the centre of the cake, if it is thoroughly cooked, the skewer should come out clean.
Once baked turn out the sponge, remove the baking paper and leave to cool on a wire rack.
Place all your ingredients for the topping into a bowl and whisk thoroughly with an electric mixer / hand mixer.
Spread evenly over the cake using a palette knife or back of a spoon. Sprinkle any left over walnuts to decorate.
Leave to harden slightly in a cool place and then serve.
