Place the cornstarch and 1/4 cup sugar in a medium bowl and mix together well; add egg yolks and mix until a paste is formed. Stir in 1/2 cup milk. Place remaining 1/4 cup sugar and water in a small heavy pot and stir to combine. Cook over medium heat, swirling the pan occasionally but not stirring, until sugar turns a medium amber. Carefully pour in remaining milk and stir until smooth, Stir in vanilla seeds. Bring mixture back to a simmer then pour hot mixture over mixture in bowl, whisking constantly. Pour back into saucepan. Cook over moderate heat, stirring constantly, until smooth and thick. Remove from heat and stir an additional minute. Transfer to a bowl. Cover with buttered parchment or plastic wrap touching top of cream to avoid skin formation. Chill a minimum of 2 hours or as long as 2 days.