Instructions

for the blondies

Adjust oven rack to middle position and heat oven to 350 degrees. Lightly grease two 9-inch round cake pans, line with parchment paper, and grease parchment. Whisk 1½ cups (7½ ounces) all-purpose flour, 1½ teaspoons pumpkin pie spice, 1 teaspoon baking powder, and ½ teaspoon table salt together in medium bowl.
Whisk 12 tablespoons melted and cooled unsalted butter, 1½ cups packed (10½ ounces) light brown sugar, ¼ cup canned unsweetened pumpkin puree, and ¼ cup light corn syrup together in large bowl. Add 2 lightly beaten large eggs and 4 teaspoons vanilla extract and whisk until smooth. Using silicone spatula, stir in flour mixture until fully incorporated. Divide batter between prepared pans, then spread into edges of pans and smooth tops. 
Bake until tops of blondies are light golden brown and toothpick inserted in centers emerges with few moist crumbs attached, 16 to 19 minutes. Let blondies cool completely in pan on wire rack, about 2 hours. Once cool, remove blondies from pans and discard parchment.

for the gingersnap crumble

Meanwhile, microwave 1 cup white baking chips in large bowl at 50 percent power, stirring occasionally, until melted, about 2 minutes. Stir in 8 ounces gingersnaps broken into rough ½- to 1-inch pieces until mostly coated. Transfer gingersnaps mixture to parchment-lined baking sheet and spread into even layer; let sit at room temperature until firm, about 30 minutes. Break 1 cup of mixture into coarse ½- to 1-inch pieces. Chop remaining mixture fine (you should have about 2½ cups); reserve coarsely broken crumble and finely chopped crumble separately.

for the pumpkin ganache

Lightly grease bottom and sides of 9-inch springform pan, line with parchment paper, and grease parchment. Microwave 2 cups white baking chips, ¾ cup canned unsweetened pumpkin puree, ¼ cup light corn syrup, 4 tablespoons butter, 1 teaspoon pumpkin pie spice, and ½ teaspoon salt in medium bowl at 50 percent power, stirring occasionally, until smooth, about 3 minutes. Pour 1 cup ganache into prepared pan and spread to edges of pan with silicone spatula. Transfer pan to freezer until ganache is firm, 30 minutes to 1 hour. (Keep remaining ganache at room temperature.) 
Thirty minutes before assembling cake, transfer ice cream to refrigerator to soften.
Working quickly and using offset spatula, spread 1½ pints (3 cups) softened vanilla ice cream over frozen ganache in even layer (top needn’t be perfectly smooth). Sprinkle evenly with half of finely chopped crumble (about 1¼ cups). Drizzle evenly with half of remaining room-temperature ganache (about ½ cup). Top with 1 blondie layer, pressing lightly to compact ice cream. Repeat layering with remaining 1½ pints (3 cups) ice cream, remaining half of finely chopped crumble, remaining half of ganache, and remaining blondie, pressing lightly to compact ice cream (cake may be slightly taller than pan; this is OK). Wrap cake tightly with plastic wrap and freeze until fully firm, at least 6 hours or up to 5 days.
Fifteen minutes before serving, unwrap cake and remove side of pan. Invert cake onto cake stand or serving platter; remove bottom of pan and parchment. Transfer cake to refrigerator to soften for 15 minutes. Arrange coarsely broken crumble around edges of cake. Slice cake, wiping knife clean between slices. Serve.