Run the paring knife down the center of the vanilla bean. Split it open with your fingers and use the knife to scrape out the tiny black seeds into a heavy-bottomed sauce pan.
Add 2 1/4 cup water, the sugar, the corn syrup and the vanilla pod and bring it to a rolling boil over medium high heat.
Remove from heat and allow the bean to steep for 15 minutes. Remove the pod, dry it, and save it for another use. If you are using vanilla extract, add it after the syrup has been removed from the heat.
Prepare the Chocolate.
Combine the chopped chocolate and cocoa powder in a medium-sized bowl. Gently whisk in 1/3 of the hot vanilla syrup. (The chocolate will begin to seize a bit.)
Whisk in the remaining syrup, in two more parts, and continue to whisk until the sauce is shiny, thick, and smooth, about 4 minutes.
Pass the chocolate sorbet base through a fine-mesh strainer to remove any lumps of cocoa powder. Place the sorbet in an ice bath to cool completely.
Transfer to the refrigerator for a minimum of 4 hours or up to 1 week.
Churn the Sorbet.
Churn the sorbet in an ice cream machine according to the machine manufacturer’s directions. The sorbet is finished once it has increased in volume and it holds whisk lines from the stirring mechanism and mounds like softly whipped cream.
Transfer to the freezer for 4 hours to attain scoopable consistency.