In a bowl, pour the condensed milk, the baking soda, the vanilla seeds and the salt. Mix vigorously.
Cook the mixture in a double saucepan on a low heat for about an hour, stirring with a whisk every now and then.
Transfer the dulce de leche to a piping bag with a decorated nozzle and keep in the fridge to harden. (*)
Make the cocoa cookies
In a bowl of a food processor or in the standing electric mixer fitted with a paddle attachment, add the butter, the oil, the sugar, the vanilla seeds and the fleur de sel. Knead until they are well combined into a creamy and homogeneous batter.
Add the egg and knead again.
Sift the cornstarch (or the potato starch), the cocoa powder and the flour and add them to the cookie dough.
If you prefer, you can also add a pinch of chili powder - but only if the cookies aren’t for children.
Knead again until well combined.
Give a round shape to the dough and cover it with cling film. Keep in the fridge for at least an hour.
Preheat the oven to 190°C.
Using a rolling pin, roll the dough between two sheets of baking paper (approximately ½ cm thick). Use a cookie cutter to cut about 20 small disks.
Cover a baking sheet with baking paper and then transfer the cookies to it and bake for about 9-12 min (depending on their size).
Allow the cocoa cookies to cool down on a wire rack before stuffing them with the dulce de leche.
Enjoy!!
NOTE
If you don’t use the dulce de leche now to stuff the cookies, put it in an airtight container and keep in the fridge for a couple of days.
Cookie recipe from "Baking a moment"
Recipe by OPSD Il gustoso food blog at https://www.opsd.it/biscotti-cacao-dulce-de-leche/