Alfajores (Dulce de Leche Sandwich Cookies)

Step 1
Mix flour, 1/2 cup confectioners' sugar, cornstarch, and salt together in a bowl.

Step 2
Beat butter with an electric mixer in a separate bowl until soft and fluffy. Mix in white sugar and vanilla until well combined.

Step 3
Add dry ingredients to wet ingredients in 3 batches, beating until just blended after each addition. Divide dough in half, roll each half into a log, and refrigerate until firm, about 2 hours.

Step 4
Preheat the oven to 350 degrees F (175 degrees C).

Step 5
Slice dough logs into forty-eight 1/4-inch-thick cookies and place onto ungreased baking sheets.

Step 6
Bake in the preheated oven until just starting to turn golden around the edges, 8 to 10 minutes.

Step 7
Remove from the oven and let cool for 1 minute on the baking sheet. Transfer cookies to wire racks to cool completely, 25 to 30 minutes.

Step 8
Use a knife to generously spread dulce de leche onto 1/2 of the cooled cookies. Sandwich remaining cookies on top and place onto a serving tray.

Step 9
Lightly dust finished cookies with remaining confectioners' sugar.