1. Preheat oven to 350°. Mix 1 1/2 cups sugar and 1 cup water in a medium saucepan. Cook without stirring over high heat until sugar turns medium brown, 15 to 20 minutes, swirling pan at end to help brown evenly. Remove from heat and carefully whisk in cream (it will splatter). Pour into a heat-proof bowl; set aside.
2. Meanwhile, grease a 10-in. metal pie pan. In a medium bowl, mix crumbs, remaining 2 tbsp. sugar, and the butter with a fork until combined. Press over bottom and up sides of pie pan. Bake until golden brown, about 5 minutes.
3. Pour half of caramel into pie pan, tilting pan so it spreads evenly over crust. Put in freezer to chill 20 minutes. Set aside remaining caramel to use when serving.
4. Soften ice cream at room temperature 10 minutes. Empty 1 carton into bowl of a stand mixer. Beat until smooth (like soft-serve), about 20 seconds. Quickly spread over caramel layer. Return to freezer, making sure it sits level.
5. Beat second carton of ice cream the same way. Add pumpkin purée and spices; blend just until combined. Spoon pumpkin ice cream over vanilla ice cream. Freeze until firm, 4 to 6 hours.
6. Let soften in refrigerator about 15 minutes. Use a warm knife to cut slices. Rewarm caramel to drizzle over top and sprinkle with pumpkin seeds if using.
Make ahead: Up to 1 week, covered in freezer.