Grease and flour a 20 cm (8 inch) square cake tin. Sift the flour, sugar, baking powder, xanthan gum and cinnamon together into a large bowl. Add the oil, eggs and vanilla extract and beat well.
Fold in the carrots, coconut, pineapple and sultanas and spoon the mixture into the prepared tin. Place in a preheated oven, 180°C (350°F), Gas Mark 4, for about 1 hour or until a skewer inserted in the middle comes out clean. Remove from the oven and cool in the tin.
Beat together the cream cheese and honey and smooth over the cake, then sprinkle the nuts, if using, on top. Cut into 16 squares.