Preheat the oven to 350 F and line a baking sheet with parchment paper.

In a medium bowl, sift the flour, cornstarch, baking powder and salt, and set aside.

In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar until smooth. Add one egg yolk at a time, followed by vanilla extract and lemon zest (if using).

Working in batches, add the sifted dry ingredients  and mix until well combined.

Transfer the dough onto a floured working surface and roll into a ball. Cover with plastic and chill for at least an hour.

Remove from the fridge and allow to come to room temperature for around 10 minutes. Knead for a couple of minutes until it’s easy to handle.

Divide the dough into 2 balls. Roll out the dough, using as little flour as possible to around 1/4 inch thick.

Cut the dough using a small round cookie cutter, and transfer to a lined baking sheet. 

Bake for 10-12 minutes. Transfer to a cooling rack and allow to cool.

Using a spoon or piping bag fill the center with dulce de leche. Dust with powdered sugar and enjoy. YUM!