Grease the bottom and sides of a 20x20 cm – 8x8 inch baking pan with butter. Line the pan with baking paper, leaving an overlap on the sides.

In a large heat proof bowl put the sweetened condensed milk, Nutella, dark chocolate chips, chopped butter and vanilla.

Cook in a double boiler, stirring, until it is smooth and all the chocolate is melted.

Once the mixture is smooth, pour it into the prepared pan and smooth the top with a spatula.

Now you have 2 options: you can sprinkle with sea salt or leave it plain. I left mine plain as I found that the salt melted after a little bit!

Refrigerate until the fudge is firm, for at least 2 hours.

Run a knife under hot water, dry it off, and run it around the edges of the pan to loosen the fudge.

Using the overhanging baking paper, lift the fudge out and peel off the baking paper. Cut the fudge into small squares.

Store in the refrigerator in an airtight container.

Sprinkle the fudge with salt just before serving it.