Beat the egg yolks in a stand mixer bowl using the whisk attachment or in a large bowl using a hand mixer together with 1/3 cup of the sugar, until thick and lemon colored.
Stir together in a medium saucepan the remaining 2/3 cup of sugar, the cornstarch, salt, heavy cream, milk, and vanilla or vanilla paste. Bring to just a boil over medium low heat, stirring continuously.
Pour about 1/3 of the milk mixture into the beaten egg yolks while whisking the two liquids together quickly with the other hand.
Pour back into the saucepan and continue cooking over medium low or low until the mixture will coat the back of a wooden spoon. You don't want to overcook this! It will only need a few minutes. Pour through a sieve to strain out any lumps.
Cool and chill overnight or 3 to 4 hours minimum. Pour into an ice cream maker and process for the time recommended (mine takes 25 minutes). Scoop into a Rubbermaid container with a lid and freeze for 2 to 4 hours for a thicker texture or serve immediately for a softer texture.
Serve with toppings: berries, decadent dark hot fudge, caramel sauce, cut up candies, sweetened whipped cream, or whatever else you like for a perfect ice cream sundae bar.