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1
Finely chop chestnuts and cook with 1 1/4 cups milk at a bare simmer in a large heavy saucepan, uncovered, stirring occasionally, 15 minutes.
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2
Puree in a blender until smooth (use caution when blending hot liquids).
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3
Bring cream and remaining 1 3/4cups milk just to a simmer in heavy saucepan and remove from heat.
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4
Beat together yolks and brown sugar in a large bowl with an electric mixer at high speed until thick and pale and mixture forms a ribbon when beaters are lifted.
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5
Whisk 1/3 of hot cream mixture into yolk mixture to temper, then whisk yolk mixture into remaining cream mixture.
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6
3Whisk in chestnut puree and salt, then add zest.
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7
Cook over moderately low heat, stirring constantly, just until mixture registers 170F on an instant-read thermometer (do not boil).
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8
Immediately pour custard through a fine sieve into a clean metal bowl and stir in vanilla.
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9
Set bowl in a larger bowl of ice and cold water and cool custard, stirring occasionally.
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10
Chill, surface covered with waxed paper, until cold, at least 3 hours.
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11
Freeze custard in ice-cream maker, then transfer to an airtight container and put in freezer to harden.
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12
Bring cream just to a simmer in a heavy saucepan over moderate heat and remove from heat.
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13
Add chocolate and butter, whisking until smooth.
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14
Whisk in Grand Marnier.
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15
Serve ice cream with sauce.
