For praline: Line rimmed baking sheet with foil. Stir sugar, 1/2 cup water, and cream of tartar in heavy medium saucepan over medium heat until sugar dissolves. Increase heat and boil without stirring until syrup is pale amber color, occasionally brushing down sides of pan with wet pastry brush and swirling pan, about 5 minutes. Add nuts; stir with wooden spoon until coated with syrup. Continue to cook until syrup is deep amber color, gently pushing nuts into syrup, about 3 minutes longer. Immediately scrape mixture out onto prepared baking sheet, spreading as thinly as possible with metal spatula. Let praline stand until cool and firm, about 1 hour. Break praline into irregular pieces. Set aside a few pieces for garnish. Coarsely chop remaining praline. (Can be made 3 days ahead. Store airtight at room temperature.)
For terrine: Whisk yolks in bowl to blend. Bring cream, milk, and sugar to simmer in heavy medium saucepan, stirring until sugar dissolves. Gradually whisk cream mixture into yolks; return to saucepan and stir constantly until mixture thickens slightly and instant-read thermometer inserted into mixture registers 170°F, about 3 minutes (do not boil). Transfer custard to 4-cup measuring cup; cool slightly.
Place white chocolate in medium metal bowl. Set bowl over saucepan of barely simmering water; stir just until chocolate is melted and smooth. Remove bowl from over water. Gradually whisk warm custard into melted chocolate. Mix in vanilla. Cover and chill custard at least 4 hours and up to 1 day.
Process custard in ice cream maker. Transfer ice cream to container; cover and freeze at least 6 hours or overnight.
Line 9x5x3-inch loaf pan with double layer of plastic wrap, leaving 4-inch overhang. Spread 1/3 of ice cream over bottom of prepared loaf pan; sprinkle with half of chopped praline. Spread half of remaining ice cream over; sprinkle with remaining chopped praline. Spread remaining ice cream over. Fold plastic wrap over to cover. Freeze overnight. (Can be made 3 days ahead. Keep frozen.)
Invert terrine onto platter; remove plastic wrap. Serve terrine with sauce and garnish with reserved praline pieces.