In a large saucepan over medium heat, bring 1.5 cups of the cream and the butter just to a simmer, stirring, until the butter is melted.
Remove from the heat and add the sweetener, chocolate, vanilla extract, and salt. Let sit for about 5 minutes, until the chocolate is melted, then whisk until smooth. Let the mixture cool to room temperature.
Whisk the remaining 1 cup of cream and the vodka, if using. Refrigerate until just cool to the touch, 1 to 2 hours.
Pour into an ice cream maker and churn according to the manufacturer's instructions. Transfer to an airtight container and freeze until firm, another 2 hours or so.