Grease a deep 23cm round cake tin, line the base with baking paper.
Cream butter, rinds and sugar until light and fluffy, add egg yolks; beat until combined.
Stir in half the sifted flour with half the cold pumpkin, then stir in the remaining flour and pumpkin.
Beat the egg whites until soft peaks form, fold through the cake mixture. Spread mixture into cake tin.
Bake in a moderate oven about 1 hour or until cooked when tested with skewer.
Pour hot syrup over hot cake and let stand 10 minutes before turning out onto a rack to cool.
To make syrup:- Combine all ingredients in saucepan.
Stir constantly over heat without boiling until sugar is dissolved.
Bring to boil, reduce heat, simmer 2 minutes without stirring.