02Whisk 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.
03Place 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.
04Process custard in ice cream maker. Transfer ice cream to container; cover and freeze at least 6 hours or overnight.
05Line 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.)
06Invert terrine onto platter; remove plastic wrap. Serve terrine with sauce and garnish with reserved praline pieces.