1Heat oven to 350°F (325°F for dark or nonstick pan). Grease bottom only of (8- or 9-inch) square pan. Make and bake cake as directed on box using water, butter, vanilla and eggs. Run knife around sides of pan to loosen cake. Cool 1 hour. Cut cake into 1-inch pieces; set aside.
2Meanwhile, in heavy 2-quart heavy saucepan, stir together sugar and cornstarch; stir in milk. Cook and stir over medium heat until thickened and bubbly. In medium bowl, beat eggs with wire whisk just until blended. Gradually stir 1 cup hot milk mixture into egg mixture, whisking constantly, until combined. Pour milk and egg mixture back into saucepan. Return to medium heat. Cook, stirring constantly, 1 to 2 minutes longer, or until thick and bubbly. Remove from heat. Stir in butter and vanilla. Press plastic wrap on filling to prevent a tough layer from forming on top. Refrigerate at least 1 hour.
3In medium bowl, whip cream until stiff peaks form. Fold whipped cream into pudding mixture until combined.
4In large bowl or trifle bowl, place half of the cake pieces. Brush or sprinkle half the liqueur over cake. Spoon half the berries, half the mangos and half the coconut over cake. Spread half the pudding mixture over coconut. Repeat with remaining cake, liqueur, berries, mangos and coconut. Cover; refrigerate at least 1 hour before serving. Garnish with additional fruit and coconut, if desired. Refrigerate any remaining trifle.