Beat the soft butter together with the sugar until the texture becomes fluffy.
Add the melted and cooled chocolate, vanilla, and egg yolk, mix until well combined.
Sift the flour, cocoa powder, mix again, and form a ball.
Put the dough in the refrigerator for 30 minutes.
Form balls, transfer to a baking tray lined with baking paper, make a depression in the middle of each ball, bake for 12-14 minutes, temperature 160 °C.
Cool completely.
Fill each indentation with boiled condensed milk, sprinkle with salt if desired.
Enjoy!
