Method

• 1.
  Combine both chocolates (reserving extra 100g) in a heatproof bowl over a saucepan of just-simmering water (make sure base of bowl doesn’t touch the water), stirring occasionally until just melted.
• 2.
  Meanwhile, combine condensed milk, butter, glucose and remaining 50ml whisky in a medium saucepan over low heat, stirring occasionally until melted and smooth (mixture should be about the same temperature as the melted chocolate).
• 3.
  Cool slightly, if needed, before combining. Add the condensed milk mixture to the melted chocolate in three batches, stirring each to incorporate before adding the next – don’t worry if the mixture starts to look split, it will come together. Add 1 tsp sea salt flakes, then beat the mixture vigorously with a wooden spoon for about 5 minutes – the mixture will look oily to begin with, but will come together in a beautiful glossy, almost elastic texture. Beat for another minute to thicken.
• 4.
  Spread one-third of the fudge into the prepared tin. Scatter with one-third of the shortbread and one-third of the extra chocolate and extra salt flakes, if you like. Repeat layers twice more.
• 5.
  Refrigerate overnight to firm. Place fudge in the freezer for 30 minutes to make it easier to cut, then remove from pan and cut into pieces with a hot, wet knife. Store in an airtight container in the fridge for up to 2 weeks.