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1
Put oven rack in middle position and preheat oven to 350F
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2
Toast hazelnuts in a shallow baking pan in oven until skins split and nuts are pale golden, about 10 minutes.
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3
Remove from oven (turn oven off), then wrap hazelnuts in a kitchen towel and rub to remove any loose skins.
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4
Cool nuts completely.
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5
Pulse nuts with granulated sugar in a food processor until finely chopped.
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6
Melt chocolate in top of a double boiler, stirring until smooth.
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7
Remove bowl from heat and set aside.
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8
Sieve flour, cocoa powder, baking powder, and salt in a bowl.
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9
Beat butter and brown sugar in another bowl with an electric mixer at medium-high speed until creamy, about 3 minutes.
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10
Add eggs 1 at a time, beating well after each addition, then beat in melted chocolate until combined.
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11
Add milk and vanilla, beating to incorporate.
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12
Reduce speed to low and add flour mixture, mixing until just combined.
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13
Stir in nut mixture.
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14
Cover bowl with plastic wrap and chill dough until firm, 2 to 3 hours.
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15
Put oven racks in upper and lower thirds of oven and preheat oven to 350F Line 2 large baking sheets with parchment paper.
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16
Sift confectioners sugar into a bowl.
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17
Halve dough and chill 1 half, wrapped in plastic wrap.
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18
Roll remaining half into 1-inch balls, placing them on a sheet of wax paper as rolled.
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19
Roll balls, 3 or 4 at a time, in confectioners sugar to coat generously and arrange 2 inches apart on lined baking sheets.
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20
Bake, switching position of sheets halfway through baking, until cookies are puffed and cracked and edges feel dry (but centers are still slightly soft), 12 to 18 minutes total.
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21
Transfer cookies (still on parchment) to racks to cool completely.
