Heat oven to 350°. Coat a 13 x 9 x 2-inch baking pan with nonstick cooking spray. Line bottom with wax paper; coat paper with spray. Prepare cake mix as per package directions with eggs, oil and 1 1/4 cups water. Bake at 350° for 35 minutes or until cake springs back when pressed. Cool 10 minutes in pan on a wire rack; invert directly onto rack, remove paper and cool completely.
While cake cools, in a large metal bowl, combine egg yolks, 1/2 cup sugar, 1/4 cup water and butter. Place bowl over a pot of simmering water. Cook, whisking constantly, until mixture is pale and thickened and registers 160° on an instant-read thermometer, about 7 minutes. Remove bowl from saucepan and whisk in chocolate until smooth. In a separate bowl, beat 1 cup of the cream with 2 tbsp of the sugar and the vanilla until mediumfirm peaks form. Fold whipped cream into chocolate mixture until no white remains. Cover surface directly with plastic and refrigerate at least 1 hour.
Once cake has cooled, cut into 1 1/2-inch cubes. Whip remaining 1 1/2 cups cream with remaining 3 tbsp sugar to medium-firm peaks. Begin layering trifle: Spoon half the cake cubes into a 4-quart glass bowl or dish. Compress slightly. Spread half the chocolate mousse (about 1 1/2 cups) over cake cubes. Top mousse with 1 pkg of the raspberries and half the whipped cream (about 1 1/2 cups). Repeat layering and garnish with chocolate curls or cocoa powder. Serve immediately or refrigerate up to overnight until serving.