Place the butter, sugar, cacao, salt and vanilla essence in the bowl where you're going to beat them later.
Place the bowl on a bain-marie without touching the surface of the water. Leave it there until everything is melted. Mix it together every couple minutes.
Remove the bowl from the bain marie and wait 5min for it to cool down a bit.
Mix using a stand or hand-held mixer and add the eggs one at a time. Mix until fully incorporated.
Add the flour and mix for 3 min on high speed.
If the batter looks liquidy, place it in the fridge for an hour.
Use a cookie scoop (like an ice cream scoop, only smaller) to place the dough on the tray lined with silpat or baking paper. You can also use a generous tablespoon.
Take to a preheated oven at 180C for 8-10min. When you take them out they'll be really soft and you think they need more time but they're ready!
Push the chocolate chunks or chips down onto the cookie. They will melt with its heat.
Sprinkle salt on the cookies.
Wait for them to cool down for 10min and then remove them from the tray.