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Heat the oven to 180C/160C fan/gas 4. Oil the base and sides of a 23 x 32cm baking tray and line with baking parchment. Beat the egg whites, lemon juice and a pinch of salt together using a stand mixer or electric whisk until very stiff. Add 1 tbsp of the sugar and beat to stiff peaks, then add the rest of the sugar, a spoonful at a time, until the mixture is thick and shiny. Sift over the cocoa and cornflour, then gently fold these in with a spatula until completely combined. Spread the mixture evenly into the prepared tin, then bake for 14-15 mins until just crisp to the touch. Leave to cool completely in the tin.
Meanwhile, make the fillings. For the chocolate filling, tip the chocolate and cream into a heatproof bowl set over a pan of simmering water and stir until the chocolate has melted and the mixture is shiny and smooth. Remove from the heat and leave to cool to room temperature. For the cream filling, whisk the cream and sugar together to stiff peaks using an electric whisk.
Dust a large sheet of baking parchment with cocoa powder, then carefully flip the meringue onto it. Remove the tin and carefully peel away the parchment. With a short end facing you, score a line across the meringue, 2cm in from the edge. Spread over the chocolate filling followed by the cream, then drizzle with most of the caramel sauce. Sprinkle over the sea salt flakes and drag a palette or cutlery knife through the fillings to gently swirl them. Roll up the roulade into a tight log, starting at the scored end and using the parchment underneath to help you. Lift onto a platter or board. Will keep chilled for up to a day. Drizzle over the remaining caramel sauce, then finely grate over more chocolate to serve.