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1
Preheat oven to 350F.
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2
Butter bottom and sides of an 8-inch square metal baking pan, then dust with cocoa powder, knocking out excess.
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3
Finely grind nuts in a food processor with flour, sugar, and salt.
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4
Melt butter in a 2-quart saucepan, then remove from heat.
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5
Add chocolate and whisk until smooth.
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6
Add eggs 1 at a time, whisking until smooth.
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7
Stir in nut flour until just combined.
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8
Spread batter evenly in baking pan and bake in middle of oven until top is firm and dry and a tester inserted into center comes out with crumbs adhering, 15 to 20 minutes.
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9
Cool completely in pan on a rack, at least 2 hours.
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10
Lightly beat yolks in a bowl.
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11
Bring cream with salt just to a boil in a small heavy saucepan, then add half of it to yolks in a slow stream, whisking constantly.
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12
Whisk yolk mixture into remaining cream and cook over low heat, whisking, until slightly thickened and an instant-read thermometer registers 170F (do not let boil).
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13
Remove from heat and add chocolate, whisking until smooth.
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14
Pour ganache over hazelnut base in pan, smoothing top with offset spatula, and chill, covered, until firm, at least 5 hours.
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15
Cut into squares with a warmed thin knife, wiping off knife after each cut, and remove from pan while still cold.
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16
Serve truffles cold or at room temperature.
