Heat milk in a small saucepan over medium heat until just hot. Meanwhile, beat sugar & eggs together in a medium bowl until thick & pale yellow.
Gradually whisk hot milk into egg mixture in bowl, then pour milk-egg mixture into saucepan. Return saucepan to medium-low heat and cook custard, stirring constantly with a wooden spoon until it is thick enough to coat the back of the spoon, about 5 minutes. Remove saucepan from the heat and stir in half the peanut butter. Strain the custard inot a large bowl, let cool briefly, stirring often, then stir in cream and vanilla. Cover & refrigerate until cold, 6-8 hours.
Process mixture in an ice cream maker according to manufacturer's directions, adding half the chopped nuts to the ice cream 30 seconds before it has finished churning. Transfer to a bowl, quickly swirl in remaining peanut butter, cover, and freeze until hard. Serve ice cream sprinkled with remaining nuts.