Slice the vanilla bean in half lengthwise and scrape the seeds (do not discard), set aside.
Roughly chop the chocolate and melt in a medium sauce pan together with milk and cream (over medium heat). After the chocolate has melted, whisk in the cocoa powder, avoiding clumps. Add the vanilla bean and seeds. Bring the mix to just under a boil (150° on a candy thermometer), stirring frequently. Remove from heat and let the mix steep, covered, for 30 minutes to an hour.
Remove the vanilla bean. Reheat mix over moderate heat until hot (150 degrees), stirring frequently.
Whisk the eggs, sugar and salt together in a small bowl.
Temporarily remove the pan from the heat. Add 1/2 cup hot cream mixture to the eggs/sugar in a slow stream, whisking constantly. Whisk the egg/cream mix back into the saucepan and return to medium-low heat, STIRRING CONSTANTLY until a candy thermometer reads 170 degrees (do not let it boil).
In a medium to large bowl (that cream mixture will be chilled in) combine the cinnamon and cayenne pepper. Pour a small amount of the chocolate cream through a strainer into the bowl and whisk to combine, avoiding clumps. Pour the remaining chocolate cream mixture through the strainer and whisk to combine.
Cool completely in the refrigerator (at least 4 hours). Freeze in an ice cream maker. Transfer to an airtight container and put in freezer to harden.
Notes: DO NOT add more than 1/8 tsp cayenne pepper. The flavor intensifies as it freezes in the ice cream maker. If you don’t have 1/8 tsp, use 1/4 tsp and remove half the cayenne.
Notes: To chill more quickly, place the final mixture in an ice and cold-water bath for 30 minutes (a larger bowl for the ice water, a smaller bowl filled with the cream mixture that will sit in the "bath"). Stir frequently.