Butternut Bread

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
2 eggs
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. Pur