Take a double base saucepan and pour in the condensed milk along with chopped bittersweet chocolate (this case you can add them whole or chop it to little pieces) the latter makes it melt faster though

Turn on the stove, bring it to low heat and place the sauce pan on top, let the mixture sit there for a minute to incorporate with each other then stir till the chocolate melts well combine with the condensed milk

Next, once it's all melted and well combined add cocoa powder, vanilla extract and salt. You can add orange or lemon zest, and dry fruits (optional)

Mix together till it all thickens

Remove from heat

Line a 8x8 baking pan with cling wrap or foil (or you don't have to if you have a non stick)

Then pour the mixture into it, smoothen it out evenly with a back of a spoon or spatula.

Sprinkle it with any kind of topping you like and put it into the fridge, leave it to set for 1 hour or overnight (whichever you prefer)

A little tip: make sure to use a thick condensed milk, so that it will make sure the mixture actually thickens hard, not runny instead