Heat the Milk Mixture. Whisk together milk, vanilla extract, and sugar in a saucepan; heat until steaming (not boiling).
1 1/2 cups milk, 1 teaspoon vanilla extract, 7 tablespoons sugar
Prep the Yolks. In a separate bowl, whisk egg yolks, cornstarch, and 2 tablespoons of the lukewarm milk mixture until smooth.
4 egg yolks, 3 1/3 tablespoons cornstarch
Temper the Eggs. Slowly whisk a few spoonfuls of hot milk into the yolk mixture, repeating 3–4 times to prevent curdling.
Cook Until Thick. Pour everything back into the saucepan; cook over medium heat, stirring constantly, until thick and bubbling (2–4 minutes).
Remove and Whisk. Take off heat and whisk for about a minute to smooth out any lumps.
Stir in Peanut Butter. Add creamy peanut butter and stir until fully blended.
3/8 cup peanut butter
Taste and Adjust. Stir in salt, then taste and add more salt or sugar if needed.
1/8 teaspoon salt
Strain if Desired. For extra smoothness, press through a fine-mesh sieve.
Cover and Chill. Lay plastic wrap on the surface to prevent a skin; cool briefly at room temperature, then refrigerate.
Whisk and Use. Once chilled, whisk until smooth and use for piping or spreading.