For custard, in a heavy medium saucepan, combine sugar (if using), cornstarch, and salt. Stir in milk. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 2 minutes more. Remove from heat.
Gradually whisk about half of the hot mixture into the beaten egg yolks. Return all of the egg yolk mixture to the saucepan. Cook and stir over medium heat just until mixture is bubbly. Remove from heat. Quickly cool the custard by placing the saucepan into a bowl half-filled with ice water for 3 minutes, stirring constantly. Strain mixture through a fine mesh sieve into a medium bowl. Stir in sugar substitute (if using) and vanilla. Cover the surface with plastic wrap. Chill for 2 to 24 hours.
To serve, divide berries among six dessert dishes. Spoon custard over berries.
Tip: We recommend Splenda® granular, Equal® packets, Equal® Spoonful, Sweet 'n Low® packets, or Sweet 'n Low® bulk. Be sure to use package directions to determine product amount equivalent to ¼ cup sugar.
To make ahead: Prepare custard as directed through Step 3. Cover and chill for up to 24 hours. Serve as directed.