Beat soft butter with sugar until creamy, add yolk and cream, mix well.
Add flour, cocoa and salt, all sifted.Mix well, you will end with a crumbly dough, press it with hands to keep it together. Take small pieces of dough, the size of a big walnut.Roll them between hands then roll them in caster sugar to coat.
Put the cookies on a baking tray lined with parchment paper and press gently the centre of the cookie with the bottom of a wooden spoon or with your thumb.
Bake at 350°F/ 175°C for 10-15 minutes then remove from oven.If the thumbprint is less evident press a little more with the wooden spoon while cookies are still hot.
Let them cool completely before removing them from the tray and filling them with dulce de leche.
Due to the filling these cookies have to be consumed in a couple of days.