Soak sultanas in rum overnight, then drain and reserve both separately.
Bake halved, deseeded squash at 170°C for approximately 1 hour until soft.
Scoop out flesh and dry in a saucepan over low heat for 15-45 minutes.
Purée the pulp in a food processor, evaporate remaining moisture in a pan, and cool.
Preheat the oven to 180°C.
Heat 200g sugar with reserved rum and orange juice until dissolved.
Butter and flour a 23cm spring-form tin and toss dried fruits, nuts, and grated squash with 1 tbsp flour.
Whisk butter with 400g sugar until fluffy, then beat in eggs individually.
Fold in 225g of the squash purée and the spices.
Sift the remaining flour with baking powder and salt.
Mix bicarbonate of soda into the buttermilk.
Alternately stir flour and buttermilk into the purée mixture in three batches.
Fold the dried fruit and nut mixture into the cake batter.
Transfer to the tin and bake for 90 to 120 minutes until a toothpick comes out clean.
Rest the cake for 15 minutes before removing from the tin.
Prick the warm cake, drizzle with rum syrup, cool, and serve with icing sugar and whipped cream.