Blend sugar, cornstarch, and salt in a heavy large saucepan. Beat together the egg yolks and milk till smooth. Gradually stir into the sugar mixture. Cook over medium heat, stirring constantly, till mixture thickens and boils.

Boil and stir for at least one minute. Remove from heat; stir in butter and vanilla. Cover with plastic wrap and place in the refrigerator to cool completely.

To assemble trifle: Cut the pound cake into cubes and arrange half of them in the bottom of a glass bowl or trifle dish.

Top with half the bananas and half of the sliced strawberries. Spread half of the banana pudding over the berries. Top with the remaining cake cubes, bananas, and strawberries.

Spread the remaining pudding over the berries, making sure to seal the edges.

Whip the cream till soft peaks form. Beat in powdered sugar and vanilla and continue beating till stiff peaks form. Spoon over the top of the pudding.

Garnish with fresh strawberries if desired. Serve within a couple of hours.