Step 1 In a medium heavy-bottom saucepan, combine the cream and milk. Halve the vanilla bean and scrape out the seeds; add the seeds and pod to the pan. Bring to a boil, then immediately remove from the heat. Stir in the sugar until dissolved.
Step 2 In a medium bowl, briefly whisk together the egg yolks. Pour about half a cup of the hot cream mixture into the bowl, whisking constantly to warm the eggs (this helps to keep them from curdling when added to the pan). Pour the cream-yolk mixture back into the saucepan with the rest of the hot cream, whisking constantly.
Step 3Cook the mixture over medium-low heat, stirring constantly with a wooden spoon, until it has thickened to a custard and coats the back of the spoon, about 7 minutes.
Step 4Strain the custard through a fine mesh sieve into a large bowl. Whisk one-half cup of the hazelnut-chocolate spread into the hot custard until thoroughly combined. Cover and cool to room temperature, then chill in the refrigerator at least 2 hours, preferably overnight.
Step 5Freeze the custard in an ice cream maker according to the manufacturer's instructions. When the ice cream has reached a thickened "soft-serve" or frozen yogurt consistency, spoon in the remaining one-half cup spread and blend very briefly to form swirls in the ice cream. Chill in the freezer until ready to serve.