Oil a 900g loaf tin and line with a double layer of baking parchment. Put the cream, cocoa, 200g caramel and a pinch of salt in a pan set over a medium heat and bring to the boil, whisking occasionally to combine. Tip the chocolate into a medium heatproof bowl and pour over the hot cream mixture. Leave to sit for 1 min, then stir everything together until the chocolate is melted and smooth. Leave to cool slightly for 5-10 mins.
Beat the mascarpone into the cooled chocolate mixture using an electric whisk for 3-4 mins until smooth, glossy and the whisk leaves a trail behind in the mixture.
To assemble the terrine, press the top halves of the caramel wafers into the bottom of the tin, cut-side up, trimming to fit if needed.
Beat the remaining caramel with a whisk to loosen it, then spread half over the wafers in the tin. Spoon half the chocolate mixture over the top, then arrange the remaining wafer halves over the chocolate layer. Cover with the remaining caramel, ensuring everything is evenly covered, then top with the rest of the chocolate mixture. Cover the terrine with a strip of baking parchment, then freeze for at least 6 hrs, or overnight.
Remove the terrine from the freezer 10 mins before serving, then turn out onto a chilled dish or serving plate. Dust generously with cocoa powder before cutting into slices.