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1
Whisk the milk and cocoa powder together in a small heavy saucepan until blended.
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2
Set over low heat and cook, stirring constantly, until the cocoa is completely dissolved.
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3
Whisk the egg yolks and sugar together in a second small heavy saucepan until thick and lemon-colored.
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4
Ladle the hot cocoa mixture a little at a time into the egg mixture, whisking constantly until well blended.
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5
Set the saucepan over low heat and cook, stirring constantly, until dense and creamy, about 3 minutes.
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6
Strain into a bowl and cool to room temperature, covered with plastic wrap pressed directly to the surface.
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7
If you have an ice-cream maker, freeze the chocolate mixture in it according to manufacturers directions.
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8
If not, set the bowl on the freezer shelf and let it sit, stirring occasionally, until frozen but still soft and moldable, from 2 to 3 hours.
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9
(You can still-freeze the ice cream like this up to a day in advance.
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10
Let it sit at room temperature until softened, about 10 minutes, before continuing.)
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11
Using a 4-ounce ice-cream scoop, scoop up a ball of the chocolate ice cream and poke a hazelnut into the center while the ice cream is still in the scoop.
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12
Drop the truffles onto a wax-paper-lined baking sheet.
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13
Repeat with the remaining ice cream and hazelnuts, setting the formed truffles and the ice cream in the freezer if they begin to melt.
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14
Set the truffles on a level freezer shelf and freeze until firm, about 30 minutes.
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15
Meanwhile, chop the remaining hazelnuts to the size of the chopped chocolate.
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16
Toss the chopped hazelnuts and the chocolate together on a separate baking sheet.
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17
Take the truffles out of the freezer and roll them one at a time in the chopped chocolate and hazelnuts until they are lightly coated.
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18
Return to the wax-paper-lined tray, and when all are coated, return the tray to the freezer.
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19
(Depending on the temperature and how quickly the truffles start to melt, you may have to do them in batches.)
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20
Freeze at least 2 hours or up to 2 days.
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21
If keeping the truffles more than a few hours, transfer them to a covered container once they are frozen solid.
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22
To serve, let the truffles stand at room temperature until softened, about 5 minutes, before serving.
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23
Meanwhile, whip the heavy cream in a chilled bowl until it holds stiff peaks.
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24
Divide the whipped cream among chilled plates, mounding it in the center.
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25
Top with truffles and serve immediately.
