Make ice cream: Split vanilla bean lengthwise and scrape out seeds. Put seeds in a small saucepan with milk, sugar, and salt. Heat over medium-high heat, stirring often, until small bubbles form around edge, 4 to 6 minutes. Remove from heat, cover, and steep 30 minutes.
Whisk egg yolks in a medium bowl. Slowly pour in milk mixture, whisking constantly. Pour mixture back into saucepan and cook over medium heat, stirring constantly with a heatproof spatula, until custard coats a metal spoon and a finger pulled across custard on spoon leaves a path, 5 to 8 minutes.
Remove from heat and whisk in cream and crème fraîche. Pour mixture through a fine-mesh strainer into a bowl. Set bowl in a bowl of ice and a little water until cold.
Freeze mixture in an ice cream maker according to manufacturer's instructions. Transfer to an airtight container and freeze until firm enough to scoop, at least 3 hours.
Make walnuts: Line a baking sheet with parchment paper. Heat a medium frying pan over medium heat. Add sugar, butter, salt, and walnuts and reduce heat to medium-low. Stir constantly as sugar melts, turns golden brown, and coats each nut evenly, about 10 minutes. Pour nuts onto lined baking sheet. Let cool, then roughly chop.
Scoop ice cream into bowls. Drizzle with saba and sprinkle with nuts.
*Find at gourmet grocery stores and online.
Make ahead: The ice cream, up to 3 days, frozen; and nuts up to 1 day, stored airtight.