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1
Line a baking sheet with parchment paper or a nonstick baking mat (such as Silpat).
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2
Sift together flour and confectioners sugar.
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3
In a food processor, pulse together flour mixture, sugar, and butter until the mixture resembles coarse meal, about 20 seconds.
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4
With machine running, pour in the water in a slow stream, and process just until the dough comes together, about 20 seconds.
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5
Form the dough into two flattened disks and wrap well in plastic.
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6
Refrigerate for 1 hour.
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7
Preheat oven to 350F.
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8
On a well-floured work surface, roll out one disk of dough to a scant 1/4-inch thickness.
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9
Using a 1 3/4-inch round cookie cutter, cut out rounds from the dough and transfer to the prepared baking sheet.
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10
Repeat with the other disk of dough.
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11
Gather up the scraps from both batches, and reroll and cut.
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12
Sprinkle half the rounds with sanding sugar.
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13
Bake until golden brown, about 15 minutes, rotating sheets halfway through.
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14
Transfer to a wire rack to cool completely.
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15
About 30 minutes before serving, spread 1 teaspoon of the cold dulce de leche on the bottom of the unsugared cookies.
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16
Place the sugared cookies on top to make sandwiches.
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17
Serve immediately.
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18
Unfilled cookies can be stored in an airtight container at room temperature up to 3 days.
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19
Empty milk into the top of a double boiler or a heatproof bowl over a pan of simmering water.
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20
Cover with a tight-fitting lid.
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21
Cook, stirring every 10 to 15 minutes, until the milk is thick and amber in color, about 5 hours.
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22
Remove from heat, and beat with a wooden spoon to smooth out.
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23
Transfer to a clean bowl, and refrigerate several hours or up to 3 days.
