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