Line a 20cm square baking tin with plastic wrap. Toast the hazelnuts in a dry frying pan until lightly golden. Leave the nuts to cool for a few minutes then chop and set aside.
Place the butter and coconut oil in a small saucepan. Stir over a low heat until both have melted. Remove the saucepan from the heat and add the honey, cocoa, salt and vanilla. Whisk until the mixture is smooth.
Set 3 tablespoons of chopped hazelnuts aside. Mix the remaining hazelnuts into the chocolate mixture. Tip the mixture into the prepared tin and pop in the freezer for 5 minutes.
Remove the tin from the freezer and sprinkle the reserved hazelnuts over the top. Put the tin back in the freezer for a further 20 minutes, or until the fudge has set.
Cut the fudge into squares. Store in an airtight container in the fridge.