For filling, combine sugar and cornstarch in a saucepan. Stir in milk until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat; cook and stir 2 minutes longer. Remove from the heat. Stir hot filling into egg yolk; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir for 2 minutes.
Remove from the heat. Gently stir in vanilla and 1 tablespoon butter. Transfer to a bowl. Cover and refrigerate for 1 hour or until chilled.
Meanwhile, in a large saucepan, bring water and remaining butter to a boil. Add flour all at once and stir until a smooth ball forms. Remove from the heat; let stand for 5 minutes. Add eggs, one at a time, beating well after each addition. Continue beating until mixture is smooth and shiny.
Trace an 8-in. heart on parchment paper; place on a baking sheet. Spread batter evenly over heart. Bake at 400° for 20-25 minutes or until golden brown. Remove to a wire rack to cool completely.
Fold whipped topping into chilled filling; spread over pastry. Arrange strawberries over filling. Refrigerate until serving. Just before serving, drizzle melted chocolate over berries.
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