1 (2 3/4 pound) butternut squash
2 1/2 cups all-purpose flour
2 teaspoons baking powder
1 teaspoon baking soda
1 teaspoon ground cinnamon
1/2 teaspoon ground allspice
1/2 teaspoon ground nutmeg
1/2 teaspoon ground ginger
1/2 teaspoon salt
3/4 cup unsalted butter, softened
1/2 cup granulated sugar
1/2 cup packed brown sugar
1 cup sour cream
1 teaspoon vanilla extract
Trim ends off the squash and halve it lengthwise. Scoop out and discard the seeds. Cut the squash into 3-inch pieces, leaving the skin on, and steam them until completely tender, about 30 minutes. Drain well and set aside. When the squash is cool enough to handle, use a spoon to scoop flesh away from the skin. Purée the flesh in a food processor until completely smooth. Set aside.
Preheat oven to 350 degrees F. Lightly grease a 9 x 5-inch loaf pan. Line the bottom with wax paper, then grease the paper. Lightly dust pan with flour and shake out excess.
Sift flour, baking powder, baking soda, cinnamon, allspice, nutmeg, ginger and salt together in a bowl and set aside.
In another bowl, cream the butter and both sugars with an electric mixer until smooth. Add eggs, one at a time, mixing well after each addition. Add sour cream, vanilla extract and 1 cup of the reserved butternut squash purée; mix well. (Reserve remaining squash purée for another use.) Stir in reserved dry ingredients until just combined. Spread batter in the prepared pan and bake in the center of the oven until a wooden pick inserted in the center of the loaf comes out clean, about 1 hour and 15 minutes.
Cool the loaf in the pan on a wire rack for 15 minutes. Then run a knife around the edges of the pan to loosen the loaf and turn it out onto the rack. Peel off the wax paper and allow the loaf to cool completely before slicing.
Slice the bread 3/4-inch thick with a serrated knife.