Eggnog Alfajores

In a medium bowl, whisk together the cornstarch, flour, baking powder, salt, nutmeg and cinnamon.
In the bowl of a stand mixer or in a bowl with beaters, beat the softened butter and sugar together for 3-4 minutes, until light and fluffy.
Add the egg yolks, vanilla and milk and beat to fully incorporate.
Add the dry ingredients in three parts, mixing slowly just until combined between each addition.
Form dough into a disc and wrap in plastic. Refrigerate for one hour.

After chilling, roll the dough between two sheets of plastic or parchment paper until it is ¼-thick.
Punch out as many cookies as you can with a 2-inch round cutter. Place the rounds onto parchment or silicone baking mat-lined baking trays. Place in the freezer for at least 10 minutes to freeze solid before baking.
When ready to bake, preheat the oven to 350°F.
Bake the cookies for 11-14 minutes. They should not take on a lot of color. Do not overbake.
Let cool completely on a wire rack.

Transfer the dulce de leche to a piping bag fitted with a plain round tip. Pipe about a half of a tablespoon of dulce de leche onto the bottom of a cookie. Place another cookie bottom down onto the dulce de leche to sandwich the two cookies together. Repeat until all cookies are sandwiched.
Sift the powdered sugar over the tops of the alfajores, if desired.