Preheat oven to 160°C
Roast the hazelnuts for 15 minutes more or less, until golden under the skin. Remove the skin by rubbing. You can buy hazelnuts already roasted but unsalted.
If using walnuts, lightly roast them. Cut the hazelnuts in half.
Melt the chocolate in a saucepan over a bain-marie (double boiler).
When the chocolate is halfway melted, add the butter and sweetened condensed milk. Mix well until smooth.
Chocolate, condensed milk and butter are smooth and shiny.
Remove from the heat and add the hazelnuts and a pinch of salt. Mix well.
Pour into the moulds. I put Rhodoïd on the edges of the frames to make them easier to unmould.
Refrigerate overnight.
The next day, cut into small squares. You can store them in an airtight tin or wrap each square in cellophane.