Whisk together the flour, cornstarch, baking powder and salt. Set aside.
In the bowl of a stand mixer (or using a hand mixer), beat butter and confectioners’ sugar on medium-high speed until well combined. Mix in the egg yolks and vanilla until incorporated. Scrape down the sides and bottom of the bowl as necessary.
Add dry ingredients to wet ingredients and mix until combined.
Divide the dough in half and form into two discs. Wrap in plastic wrap and chill for 45 minutes.
Preheat oven to 350℉. Line two baking sheets with parchment paper and set aside.
Take one of the discs out of the refrigerator. On a lightly floured surface, roll the dough until ¼-inch thick. Use a small 2-inch round cookie cutter to cut circles out of the dough. Place on the prepared baking sheet, about 2 inches apart. Repeat with second disc of dough. Reroll scraps of dough as needed.
Bake for 8 to 10 minutes or until the edges are starting to turn golden. Cool completely.
On the bottom of one cookie spoon or pipe about 2 teaspoons of dulce de leche. Top with another cookie. Repeat with remaining cookies. Dust with confectioners’ sugar before serving.
Notes
Dulce de leche is located in the baking aisle or ethnic foods aisle at your local grocery store.Store alfajores in an airtight container at room temperature or in the refrigerator for up to 1 week.Freeze for up to 3 months. Wrap stacks of 4 or 5 cookies in plastic wrap and place in an airtight container. Freeze in an airtight container, then defrost in the refrigerator.