4 ripe peaches
1 whole vanilla bean
1 cup plus 4 tsp. sugar
1 pint vanilla ice cream, softened
1 cup raspberries
1⁄2 tsp. fresh lemon juice
Bring a large pot of water to a simmer over high heat. Lower peaches into simmering water and blanch just long enough to loosen their skins, about 1 minute. Transfer peaches with a slotted spoon to a bowl of ice water to cool, then peel, halve lengthwise, and discard pits. Put peach halves and vanilla bean into a wide dish and sprinkle with 1 cup of the sugar. Cover with plastic wrap and set aside until peach juices run and most of the sugar dissolves, about 1 hour.
Bring 3 cups water to a boil in a medium pot over high heat. Add peaches and the sugary juices and vanilla bean, reduce heat to medium, and poach, gently stirring occasionally, until peaches are just soft when pierced, 8–12 minutes. Remove pot from heat, partially cover, and set aside until peaches are cool, 1–2 hours. Transfer pot to the refrigerator until peaches are thoroughly chilled, 2–3 hours.
Meanwhile, divide ice cream between 4 wide dessert dishes, making a smooth, even layer in bottom of each dish; transfer to freezer until ice cream is solid. Purée raspberries in a blender, then strain through a sieve into a bowl, pressing purée through with a rubber spatula; discard seeds. Add the remaining 4 tsp. sugar and lemon juice to purée, stirring until sugar dissolves.
To serve, arrange 2 peach halves, pit side down, in each dish top of ice cream (save poaching liquid for another use). Spoon some of the raspberry sauce over peaches. Allow ice cream to soften slightly before serving.