Combine the flour, cornstarch, baking powder, salt, and powdered sugar.
In a large bowl add the butter and either beat it with a paddle or use a hand mixer until light and fluffy.
Add in the lemon zest, vanilla, egg, and egg yolk and mix until just combined.
Add the dry ingredients to the wet and mix until just combined. Scrape down the bowl to make sure everything is completely mixed in.
Form a large ball with all the dough and wrap tightly with plastic wrap. Transfer to the fridge and chill for 30-40 minutes.
Let the dough sit at room temp for about 5 minutes flatten to 6mm.
Use a round or scalloped-edged cookie cutter to cut out cookies.
Chill the cookies in the fridge for about 10 minutes before baking.
Preheat the oven to 190c and line a baking sheet with parchment paper.
Bake the pans of cookies on the middle rack for about 8-9 minutes, just until the edges start to turn golden. You don't want the whole cookie to be golden, just the very edges.
Transfer cookies to a cooling rack and let cool completely.
Spoon the dulce de leche into a piping bag and pipe about 1-2 teaspoons in the middle of one cookie. Sandwich a second cookie onto the first. Refrigerate until ready to serve.