• Preheat oven to 400 degrees F.

Dulce de Leche Filling

• Pour sweetened condensed milk into a oven safe dish. Add salt and cinnamon stick. Cover tightly with foil and place dish inside of a larger baking tin filled with lukewarm water. The water should cover 2/3rds of the dish. Place on middle rack and bake the milk in the water bath for 2 hours, or until the milk caramelizes to a medium brown. You may need to refill the water halfway through. Take the dish out of the oven and let it cool. Remove the foil and cinnamon stick, then use a whisk to stir the dulce de leche until it is nice and creamy. Set aside.

Cookies

• Sift together the dry ingredients (cornstarch, flour, baking powder, baking soda, and salt). Set aside.
• In the bowl of a stand mixer with the paddle attachment (or using a hand mixer), cream together butter and powered sugar until fluffy. Beat in egg yolks one at a time, making sure each egg is fully incorporated before adding the next. Add vanilla extract and mix on low until incorporated. Add lemon zest (optional).
• Add dry ingredients and mix on medium low until just incorporated. The mixture should look grainy, like wet yellow sand. Move the dough to a lightly floured surface and use your hands to press the dough until it becomes smooth (see video). Shape the dough into a ball, then cover with plastic wrap and chill for 30 minutes.
• Reduce oven temeprature to 350 degrees F.
• Remove dough from the fridge and while the dough is still wrapped in plastic wrap, use a rolling pin to roll flat, roughly 1 in high and 12 x 12. Using a 2 in round (I used a spice cap that was 4.25 cm), cut out 24 cookies. The amount of cookies you get will depend on the size cutter you use.
• Transfer cookies to a baking tray lined with parchment paper, then bake for 10-12 minutes, or until the edges begin to brown. Remove from the oven and let the cookies cool for 10 minutes before transferring to a cooling rack to cool completely.
• Fill a piping bag with dulce de leche. Fill half of the cookies with dulce de leche. Press one of the cookies without dulce de leche on top of one of the cookies with dulce, pressing gently to sandwich the cookies together. Roll in shredded cooconut (the dulce filling will stick to the coconut and keep it in place). Repeat with the remaining cookies.
• Sprinkle powdered sugar on top of the cookies and enjoy!