Grease a 20cm square brownie pan. Line with baking paper, allowing the paper to hang over the sides to make lifting out easier.
Set aside 25g of the chopped raw almonds for sprinkling at the end. Mix the remaining almonds with the chopped pecan nuts. Pour them into the prepared tin and pat into an even, single layer.
Place the butter, castor sugar, salt and vanilla extract in a saucepan. Stir over a low heat to melt the butter. Allow to boil gently while whisking continuously for 10-15 minutes until thick and lightly golden.
TIP: if the mixture gets too thick or splits take it off the heat immediately. Add 1 tablespoon of hot water and whisk vigorously until the mixture comes together. Return to the heat and continue.
Pour the hot fudge mixture straight onto the nuts and gently spread to cover them completely. Melt the dark chocolate and pour over the hot fudge. Sprinkle the chopped almonds that you set aside earlier over the melted chocolate.
This is the hardest step! Allow the fudge and melted chocolate to cool completely. (If it is a very hot day you can always pop the tray in the fridge to speed up the process.) Once cool and the chocolate and fudge have set, cut or break the slab into pieces.