On a bowl, sift together corn starch flour, all purpose flour, baking soda, and baking powder. Set aside.
In another bowl, add in icing sugar and soften butter. Mix until creamy. Add in egg yolk one by one, mixing well to combine after each addition. Add in vanilla extract, lemon zest and mix to combine.
Add in sifted flour mixture in 3 additions. Mix just until combine after each addition. For the 3rd addition, you will not be able to mix everything to combine using hand held mixer. When it's half mixed, pour the whole mixture onto the working surface. Start folding the mixture onto itself and gently push the mixture with your palm. Repeat the process until the mixture becomes a dough. Do not knead. Refer to the video for better understanding for this part.
Wrap the dough with cling film and leave it to chill in the fridge for 30 minutes.
Work with 1/3 of dough at a time to prevent the rest of dough comes to room temperature. Put 1/3 of dough on a cling film and cover it with another. Gently roll it into 8-10mm thickness. If the dough breaks, don't worry just gently push it back together. Chill in the fridge for 10 minutes while you continue rolling the rest of the dough.
Take 1 piece of rolled dough from the fridge and place it on the working surface. Remove the top cling film. Cut out circles using a 4.5 cm diameter glass. I used champagne glass. Flour the mouth of the glass to prevent the dough sticking to it.
Using a flat spatula, remove the circles onto a baking tray that has been greased and floured before hand. If you're going to cut out circles for the rest of the dough before baking them, place the already cut out circles on baking tray in the fridge.
Bake at 180ºC (360ºF) in a preheated oven for 10-12 minutes or until fully cooked without the cookies having any toasted color. Cool in a rack completely.
To assemble the alfajores, take one cookie, bottom facing up and pipe dulce de leche on it, preferably the same height as the cookie. Normal dulce de leche can be used but it doesn't hold as good. Take another cookie and put on top of the dulce de leche, bottom facing down and lightly press. Take the sandwiched cookies and roll its side in coconut flakes, with the intention of covering the dulce de leche. Repeat with the rest of the cookies. Enjoy!
* Using normal dulce de leche will make the sandwiched cookies kind of slide as it doesn't hold as good as bakery dulce de leche, which is thicker and firmer.* The alfajores taste even better the next day, if you can be that patient :D* To get that soft crumbly classic corn starch alfajores, do not over mix the dough.