1. Preheat oven to 350°.
2. To prepare cake, sprinkle hazelnut flour evenly on a rimmed baking sheet. Bake at 350° for 10 minutes or until the color of peanut butter, stirring once. Cool.
3. Combine 1/2 cup sugar, oil, and butter in a large bowl; beat with a mixer at medium speed until well blended. Add egg whites; beat until well blended. Add liqueur; beat 1 minute.
4. Weigh or lightly spoon all-purpose flour into a dry measuring cup; level with a knife. Combine hazelnut flour, all-purpose flour, baking powder, and dash of salt in a bowl. Add flour mixture to sugar mixture. Beat at low speed 1 minute. Spoon batter evenly into a 9-inch springform pan coated with baking spray. Bake at 350° for 15 minutes or until a wooden pick inserted in center comes out clean. Cool completely in pan on a wire rack.
5. Reduce oven temperature to 300°.
6. To prepare cheesecake, place cream cheese and ricotta cheese in a large bowl. Beat with a mixer at medium-high speed until smooth (about 3 minutes). Add 3/4 cup sugar and next 6 ingredients (through egg yolks); beat at low speed until smooth. Add pumpkin; stir gently until combined. Pour cheesecake batter over top of cooled cake. Bake at 300° for 1 hour or until cheesecake center barely moves when pan is touched. Remove cheesecake from oven. Run a knife around outside edge. Cool on a wire rack. Cover and chill 8 hours or overnight.
7. To prepare brittle, cover a large baking sheet with parchment paper. Draw a 14 x 2-inch area on paper; sprinkle nuts inside marked space. Place 1/3 cup sugar in a small, heavy saucepan over medium heat, and cook until sugar dissolves, stirring gently as needed to dissolve sugar evenly (about 2 minutes). Continue cooking 1 minute or until golden (do not stir). Drizzle caramelized sugar over nuts. Let stand 10 minutes or until firm; break into 14 pieces. Slice cheesecake into 14 slices; top each serving with about 1 1/2 teaspoons whipped topping and 1 piece brittle.
Nutrition Note: If you substitute an equivalent amount of whipped cream for the whipped topping to garnish, you will add 10 calories, 1.6g fat, and 1g sat fat to each serving.