In a medium bowl, combine flour, cornstarch, baking soda, baking powder, and salt. Whisk to combine.
In the bowl of a mixer, place the softened butter and the sugar. Then, mix until light and fluffy, scraping down the sides of the bowl as needed. Next, add the lemon zest, egg yolks, water, and vanilla.
Mix until combined, then slowly add the flour mixture. Mix just until it is combined, but don’t over mix.
Form dough into a disk then wrap in plastic wrap. Place in the fridge and chill until it is firm, about 1 hour.
Preheat oven to 350 degrees. Line a baking sheet with parchment paper and lightly flour a work surface.
Cut the dough in half and return unused portion to the fridge.
Roll out the half of dough you are using to 1/4 inch thickness and then cut into 12 2-inch rounds, re-rolling scraps if necessary.
Place the dough onto prepared baking sheets. Bake in the oven for 11-13 minutes, or until they are just golden on the bottom edges. The cookies will be quite pale on top, this is fine.
Remove the cookies to a cooling rack to cool completely. Repeat with remaining dough half. You will have 24 cookies after baking.
Once the cookies have cooled, turn over half of them. Then, using the back of a spoon or an offset spatula, place a tablespoon or two of the dulce de leche on the flat surface of the upside down cookie. Place a right side up cookie on top of the filling, and gently press to sandwich them together. Repeat with the remaining cookies. (You’ll have 12 sandwich cookies total.)