For pastry cream, in a large, heavy saucepan, combine egg yolks, the 1/2 cup sugar, and the cornstarch until smooth. Add milk. Cook and stir just until mixture comes to boiling. Remove from heat. Stir in butter and vanilla. Cool; cover, and chill the mixture for about 2 hours. Fold in the whipped cream.
Meanwhile, for raspberry sauce, in a 4- to 5-quart Dutch oven, heat raspberries over low heat until thawed. Stir in the 1 cup sugar. Cook, uncovered, over medium-low heat for 40 minutes or until thickened to consistency of egg whites. Cool for about 30 minutes. Sieve to remove seeds.
To layer trifle, cut sponge cake into 1-1/2-inch pieces. Begin by arranging half of the cake pieces in the bottom of a clear 3-quart glass bowl. Sprinkle 1 tablespoon sherry over cake. Spoon 1 package of strawberries over cake to cover. Repeat layers. Smooth pastry cream over dessert. Cover and chill for 2 to 24 hours.
Beat the 1 cup whipping cream with the 1 tablespoon sugar until stiff peaks form. Serve the trifle topped with the raspberry sauce, sweetened whipped cream, and chopped walnuts. Makes 10 to 12 servings.
If you like, layer ingredients in individual dessert dishes.