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1
To make the pastry, in a food processor, combine the flour, confectioners sugar, and salt and process briefly to mix.
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2
Add the butter and process until the mixture is the consistency of coarse meal, 5 to 10 seconds.
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3
With the motor running, add the egg yolk and then gradually the ice water.
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4
Process until the dough just begins to come together and will hold a shape when pressed.
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5
Transfer the dough to a floured work surface and press into a thick disk for easy rolling.
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6
Roll out into a round at least 1 1/2 inches in diameter and about 1/4 inch thick.
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7
Drape the dough round over the rolling pin, position it over a 10-inch tart pan with a removable bottom, and ease it into the bottom and sides of the pan, pressing the pastry gently into place.
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8
If using a pan with fluted sides, roll the rolling pin over the top edge of the pan with moderate pressure to trim away the excess dough.
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9
If using a pan with straight sides, raise the edges of the pastry 1/4 to 1/2 inch above the rim of the pan by squeezing the dough from both sides with your index fingers.
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10
Place the tart pan on a baking sheet.
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11
Preheat the oven to 400F
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12
To make the filling, in a food processor, finely grind the hazelnuts.
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13
Add the granulated sugar, butter, flour, and hazelnut liqueur.
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14
Pulse until a meal-like paste forms.
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15
Add the eggs and process for 10 seconds to incorporate.
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16
Spread the mixture in an even layer in the pastry-lined pan.
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17
To make the topping, arrange the plum slices in overlapping concentric circles on the filling.
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18
Be sure to fit them tightly together.
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19
Arrange 2 concentric rows of the plum slices in the center of the tart.
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20
Dot the plums with the butter and dust with the granulated sugar.
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21
Bake until the tart is browned on top, about 1 hour.
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22
Transfer to a rack and let cool completely
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23
Garnish the tart with the hazelnuts and serve.
