Let's cook.
Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a medium bowl, whisk together the all-purpose flour, cornstarch, and salt.
In a large bowl, cream together the softened butter and granulated sugar with an electric mixer until light and fluffy, about 2-3 minutes.
Beat in the vanilla extract. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined and a soft dough forms.
Roll out the dough on a lightly floured surface to about 1/8-inch thickness. Use a 1.5-inch round cookie cutter to cut out circles. Place the circles on the prepared baking sheet.
Bake for 10-12 minutes, or until the edges are lightly golden. Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.
Once cooled, spread about 1 teaspoon of dulce de leche on the flat side of one cookie. Top with another cookie to form a sandwich.
Repeat with the remaining cookies and dulce de leche. Dust the finished alfajores generously with powdered sugar before serving.
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