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1
Cut an X in bottom of each peach, then blanch in boiling water 15 seconds. Transfer with a slotted spoon to an ice bath to stop cooking. Peel peaches and cut into 1/2-inch pieces. Toss with lemon juice and 3/4 cup plus 2 Tbsp sugar in a large bowl. Let macerate, covered and chilled, at least 8 hours.
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2
Whisk together cornstarch, 1/4 tsp salt, and remaining 1/4 cup sugar in a heavy medium saucepan. Add cream and milk and bring to a boil over medium heat, whisking constantly. Add to yolks in a slow stream, whisking constantly, to temper, then pour mixture back into saucepan.
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3
Cook, stirring with a wooden spoon, just until custard coats back of spoon and registers 170u00b0F on an instant-read thermometer, 1 to 2 minutes (mixture will be thick). Strain through a fine-mesh sieve into a metal bowl and stir in extracts. Chill custard, its surface covered with parchment paper (to prevent a skin from forming), until cold, at least 4 hours.
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4
Transfer 2 cups peaches with slotted spoon to a bowl.
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5
Puree remaining peaches and liquid in a blender until smooth. Add puree to custard and freeze in ice cream maker, then transfer to a bowl and stir in reserved peaches.
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6
Transfer to an airtight container and put in freezer to harden, about 2 hours.
