Combine milk, cream, half the sugar, salt, and vanilla seeds plus pod in a saucepan. Heat over medium for 5 minutes until steaming but not boiling.

Whisk yolks and remaining sugar until pale, about 2 minutes. Slowly whisk in 1 cup of the hot milk, then return the mixture to the saucepan.

Cook over medium-low heat, stirring constantly, until the custard thickens and reaches 170°F to 175°F, 8 to 10 minutes. Strain, stir in extract if using, cool in an ice bath, then refrigerate at least 4 hours.

Churn in an ice cream maker for 20 to 25 minutes until soft-serve texture. Transfer to a container, press plastic wrap on the surface, and freeze at least 2 hours.

Let the gelato sit at room temperature 5 minutes before scooping and serving.