PreparationMake pears: Scrape seeds from vanilla bean into large saucepan; add bean. Add 6 cups water, sugar and lemon peel. Stir over medium heat until mixture comes to simmer. Add pears; simmer until tender, about 15 minutes. Cool pears in syrup. Make caramel: Stir 1 cup sugar and 1/4 cup water in heavy medium saucepan over low heat until sugar dissolves. Increase heat; boil without stirring until sugar turns deep amber, occasionally swirling pan and brushing down sides with wet pastry brush, about 10 minutes. Remove from heat. Add 1 cup warm cream (mixture will bubble up). Return to very low heat; stir until smooth. Chill sauce uncovered until cold, about 1 hour. Beat chilled cream, powdered sugar and vanilla in large bowl until firm peaks form. Fold 1/2 cup cold caramel sauce into cream; cover and chill remaining sauce. Drain pears and cut crosswise into 1/2-inch-thick slices. Press 1 piece of cake into each of six 12- to 14-ounce goblets. Top each with 5 pear slices, 2 teaspoons reserved caramel sauce and 1/4 cup caramel cream. Repeat layering with cake, pears, caramel sauce and caramel cream. (Can be made 1 day ahead. Cover and chill.) Drizzle remaining caramel sauce over trifles. Sprinkle with nuts and serve.