Preheat the oven to 190°C, 170°C fan, 340°F or gas mark 4.
Line a 20cm round shallow tin with baking paper and grease with butter.
Sieve the flour, baking powder and allspice into a mixing bowl.
Add the butternut squash, Muscovado sugar, butter, eggs, sultanas, crystallised ginger, orange juice and zest then whisk (by hand) together until blended.
Pour the batter into the tin and level with a rubber spatula.
Bake in the centre of the oven for 20 - 25 minutes until golden brown and an inserted skewer inserted comes out clean.
Allow to cool for 10 mins in tin before transferring to a cooling rack.
Beat together the icing sugar, cream cheese and orange juice to form the icing.
Once the cake is cool, use a metal spatula to spread the icing over it and decorate with orange zest shavings.