Preheat the oven to 350°F (180°C) and line a baking sheet with parchment paper.
In a medium bowl, whisk together the flour, cornstarch, baking powder, and salt.
In a large bowl, beat the butter and sugar together until light and fluffy.
Add the egg yolk and vanilla extract to the butter mixture and beat until well combined.
Gradually add the dry ingredients to the butter mixture, mixing until just combined.
On a lightly floured surface, roll out the dough to about 1/4 inch (6mm) thickness. Use a 2-inch (5cm) cookie cutter to cut out circles of dough and transfer them to the prepared baking sheet.
Bake the cookies for 12-15 minutes, or until lightly golden brown around the edges. Allow the cookies to cool completely on the baking sheet.
To assemble the alfajores, spread about 1 tablespoon of dulce de leche onto the bottom of one cookie.
Place another cookie on top and gently press down to sandwich the filling.
Repeat with the remaining cookies and filling.
Dust the tops of the cookies with powdered sugar before serving.
Notes
Tips and Tricks
Make sure the butter is at room temperature before mixing to ensure it blends smoothly with the other ingredients.Use a cookie cutter with a sharp edge to cut the dough into circles, and make sure to press the cutter all the way down to the baking sheet to ensure clean edges.If the dough is too soft to handle, refrigerate it for 30 minutes to firm it up before rolling it out.To make sure the cookies are all the same size, use a kitchen scale to weigh out each ball of dough before rolling it out.