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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Place the butter and sugar in the bowl of a stand mixer fitted with a paddle attachment. Mix on medium speed, stopping the mixer to scrape down the sides of the bowl with a rubber spatula, until the mixture is light in color and fluffy. Add the egg yolks and vanilla and mix until incorporated. On low speed, gradually add the reserved flour mixture and mix until just incorporated with no visible white pockets.

 

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Turn the dough out onto a piece of plastic wrap, shape it into a smooth disk, and wrap it tightly. Place in the refrigerator until firm, at least 1 hour.

 

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Meanwhile, heat the oven to 350°F and arrange a rack in the middle. Line 2 baking sheets with parchment paper and set aside.

 

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Remove the dough from the refrigera , unwrap it, and place it on a lightly floured work surface. Lightly flour the top of the dough. Roll to 1/4-inch thickness (the dough will crack but can be easily patched back together). 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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Place the cookies on the prepared baking sheets, 12 per sheet and at least 1/2 inch apart. Bake 1 sheet at a time until the cookies are firm and pale golden on the bottom, about 12 to 14 minutes. (The cookies will remain pale on top.) Transfer to a wire rack to cool completely.

 

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Flip half of the cookies upside down and gently spread about 2 teaspoons of Deca & Otto Dulce de Leche on each. Place a second cookie on top and gently press to create a sandwich. Dust generously with powdered sugar before serving.