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1
Preheat oven to 325u00b0F.
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2
Pulse toasted pecans with 2 tablespoons confectioners sugar in a food processor until finely ground.
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3
Whisk together flour, salt, and baking powder in a bowl.
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4
Beat together butter, remaining 2/3 cup confectioners sugar, and vanilla in a bowl with an electric mixer at high speed until pale and fluffy, about 3 minutes.
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5
Add egg yolk and beat well.
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6
Add flour and ground-pecan mixture and mix at low speed until just combined, 30 seconds to 1 minute.
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7
(Dough will be crumbly but will hold together when squeezed.) Halve dough and roll out 1 half between 2 sheets of wax paper until 1/4 inch thick (about a 9-inch round).
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8
Cut out as many rounds as possible with cookie cutter and arrange about 2 inches apart on buttered large baking sheets, reserving scraps.
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9
Roll out and cut remaining dough in same manner.
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10
Gather scraps, then reroll and cut in same manner.
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11
Beat egg white until frothy, then brush tops of rounds lightly with egg white.
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12
Put a pecan half on top of each round, then brush pecan lightly with egg white.
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13
Bake cookies in middle of oven until tops are pale golden, 15 to 20 minutes.
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14
Cool cookies on sheets on racks 2 minutes, then transfer to racks to cool completely.
