Preheat the oven to 375 degrees . Place the squash on a parchment-lined baking sheet; roast until tender, about 35 minutes. Remove and let cool, then slice in half. Discard the seeds, then scoop out the flesh until you have 1 cup.
Generously grease and flour a 9-inch springform pan. In a small bowl, add the raisins and sherry; let sit.
Lower the oven to 350 degrees . In a medium bowl, whisk the flour, baking powder and salt. In a separate bowl, beat the butter and brown sugar using an electric mixer on high speed until smooth. On low speed, mix in the maple syrup and vanilla. Add the eggs 1 at a time, beating well after each addition and scraping down the sides of the bowl. Mix in the squash, then the flour mixture 1/2 cup at a time until incorporated. Drain the raisins; add to the batter. Pour into the pan, smoothing the top.
Bake the cake until springy in the center and a cake tester inserted into the middle comes out clean, about 1 hour. Transfer to a rack; let cool 30 minutes. Remove the side of the pan, transfer the cake to a plate and dust with confectioners sugar.