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1
Place the cornstarch, measured flour, baking powder, baking soda, and salt in a medium bowl and whisk briefly to combine; set aside.
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2
Place the butter and sugar in the bowl of a stand mixer fitted with a paddle attachment.
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3
Mix on medium speed, stopping the mixer to scrape down the sides of the bowl once with a rubber spatula, until the mixture is light in color and fluffy, about 3 minutes.
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4
Add the egg yolks, pisco or brandy, and vanilla and mix until incorporated, about 30 seconds.
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5
Stop the mixer and scrape down the sides of the bowl.
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6
On low speed, gradually add the reserved flour mixture and mix until just incorporated with no visible white pockets, about 30 seconds.
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7
Turn the dough out onto a piece of plastic wrap, shape it into a smooth disk, and wrap it tightly.
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8
Place in the refrigerator until firm, at least 1 hour.
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9
Meanwhile, heat the oven to 350 degrees F and arrange a rack in the middle.
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10
Line 2 baking sheets with parchment paper and set aside.
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11
Remove the dough from the refrigerator, unwrap it, and place it on a lightly floured work surface.
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12
Lightly flour the top of the dough.
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13
Roll to 1/4-inch thickness (the dough will crack but can be easily patched back together).
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14
Stamp out 24 rounds using a plain or fluted 2-inch round cutter, rerolling the dough as necessary until all of it is gone.
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15
Place the cookies on the prepared baking sheets, 12 per sheet and at least 1/2 inch apart.
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16
Bake 1 sheet at a time until the cookies are firm and pale golden on the bottom, about 12 to 14 minutes.
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17
(The cookies will remain pale on top.)
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18
Transfer to a wire rack to cool completely.
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19
Flip half of the cookies upside down and gently spread about 2 teaspoons of the dulce de leche on each.
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20
Place a second cookie on top and gently press to create a sandwich.
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21
Dust generously with powdered sugar before serving.
