Melt the chocolate in a heatproof bowl in the microwave, stirring every 30 seconds or so to prevent burning. Alternatively, melt it in a saucepan over low heat on the stove.
Add the melted chocolate to a large bowl along with the Nutella and mix until well combined.
Transfer the mixture to your lined tin and spread it out evenly.
Let the fudge sit at room temperature for 30 minutes then sprinkle the sea salt over the top, while it's still a little soft.
Pop the fudge in the fridge for 1 hour until fully set, then cut into squares and enjoy.