Whisk together flour, baking powder, baking soda, spices and salt, set aside.
Cream together butter and brown sugar until light and fluffy. Beat in one egg at a time, incorporating after each. Whisk in sour cream, orange juice and vanilla.
Whisk in dry ingredients and combine until just incorporated.
Stir in squash and 1 cup of the hazelnuts.
Spread evenly into prepared pan and bake in preheated oven for approximately 30-35 minutes or until a toothpick inserted in the centre comes out clean. Allow to cool completely in the pan on a wire rack.
Cream Cheese Icing
Beat together butter, cream cheese and brown sugar until light and fluffy. Add vanilla and 2 cups of icing sugar, beat until smooth. Spread evenly onto cooled cake and top with remaining 1/4 cup toasted hazelnuts. Cut into 24 pieces.
Peel butternut squash then split in half lengthwise. Cut into large chunks and grate using a fine grater. It is essential to use the fine grates (like you would use to grate parmesan cheese). The finely grated squash will cook in the time the cake takes to bake.
The brown sugar in the icing will dissolve as it gets beaten with the cream cheese and butter.