Heat oven to 400°F. In 2 1/2-quart saucepan, heat water and 1/2 cup butter to rolling boil. Stir in flour. Reduce heat to low; stir vigorously over low heat about 1 minute or until mixture forms a ball. Remove from heat.
Beat in eggs, all at once, with spoon. Continue beating until smooth. Drop dough by level tablespoonfuls about 1 1/2 inches apart on ungreased cookie sheets to make 36 profiteroles.
Bake 20 to 25 minutes or until puffed and golden. Cool away from draft, about 30 minutes.
Meanwhile, in 2-quart saucepan, mix granulated sugar, cornstarch and salt. Gradually stir in half-and-half. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil and stir 1 minute. Gradually stir at least half of the hot mixture into egg yolks, then stir back into hot mixture in saucepan. Boil and stir 1 minute. Remove from heat. Stir in 2 tablespoons butter and the vanilla.
Pour filling into bowl. Press plastic wrap on filling to prevent a tough layer from forming on top. Refrigerate at least 1 hour or until cool.
Cut puffs horizontally in half. Fill puffs with filling; replace tops. Sift powdered sugar over tops. Cover; refrigerate until serving. Serve drizzled with chocolate topping. Store remaining profiteroles covered in refrigerator.