For the crust: Position an oven rack in the middle of the oven. Preheat the oven to 180°C / gas mark 4. Line the bottom of a 23cm round spring form pan with parchment or greaseproof paper. Spray the paper and the sides of the pan with cooking spray. In the bowl of a food processor, combine the digestive biscuits and sugar. Process until the mixture resembles fine bread crumbs. Add the melted butter and blend until the mixture forms into clumps. Spread the mixture into the bottom of the prepared pan, pressing gently to form an even layer. Place the pan on a baking sheet and bake for 10 to 12 minutes until the crust is golden. Cool for 15 minutes.
For the caramel: While the crust is cooling, in a medium heavy-bottomed saucepan, combine 120ml of cream, butter, sugar, and water. Stir over medium heat until the mixture is smooth. Bring the mixture to a boil and cook, without stirring, until a candy thermometer registers 115°C, about 5 to 7 minutes. Carefully pour the caramel over the warm crust. Cool for 20 minutes. Freeze until firm, about 10 minutes.
For the chocolate layer: Combine the chocolate chips and cream in a small bowl and place over a pan of simmering water. Stir until the chocolate has melted and the mixture is smooth, about 3 minutes. Whisk in the espresso powder. Remove the spring form pan from the freezer. Pour the chocolate mixture over the caramel layer and smooth with a spatula. Sprinkle the top with smoked sea salt, if using. Refrigerate for at least 1 hour until firm.
Allow the layers to come to room temperature, about 30 minutes. Using a warm, slightly wet knife, carefully cut around the edges of the chocolate layer. Release the side of the pan and remove the paper from the bottom. Cut into 4 by 1cm bars and store airtight in a covered plastic container.