- Nonstick vegetable oil spray
- 1 cup all purpose flour
- 1 teaspoon baking powder
- 1 teaspoon ground cinnamon
- 1/2 teaspoon ground ginger
- 1/2 teaspoon salt
- 1/4 teaspoon ground allspice
- 1 cup finely shredded peeled butternut squash
- 3/4 cup (packed) golden brown sugar
- 1/3 cup unsalted butter, melted
- 1 large egg
- 2 tablespoons finely grated peeled fresh ginger
- 1 1/4 teaspoons vanilla extract, divided
- 3/4 cup chopped toasted hazelnuts (about 4 ounces), divided
- 3 tablespoons whipping cream
- 3 ounces high-quality white chocolate (such as Lindt or Perugina)
- Preheat oven to 350°F. Spray 9x9x2-inch metal baking pan with nonstick spray. Whisk flour and next 5 ingredients in medium bowl. Using electric mixer, beat squash, brown sugar, butter, egg, ginger, and 1 teaspoon vanilla in large bowl to blend. Fold in flour mixture and 1/2 cup hazelnuts. Transfer to pan, spreading to edges (layer will be thin). Bake cake until tester inserted into center comes out clean, about 30 minutes. Cool.
- Bring cream just to boil in small saucepan over medium heat. Remove from heat. Add white chocolate and remaining 1/4 teaspoon vanilla; whisk until smooth. Let stand at room temperature until thick enough to spread, about 20 minutes. Spread over cake (layer will be thin). Sprinkle remaining 1/4 cup nuts over. DO AHEAD: Can be made 1 day ahead. Let stand at room temperature. Cut into 16 squares and serve.