Melt the Trex in a saucepan, then remove from the heat. Sift the flour, salt and cocoa powder into a bowl.
Add the flour mixture and fold it in very gently, using a large metal spoon. Do this carefully to retain the air that has been whisked in. Next, fold in the melted Trex.
Put a large sheet of non-stick baking parchment onto a work surface and turn the cooked cake onto it whilst still warm, so that it is upside down. Carefully peel off the lining paper. Roll up the cake, along with the fresh piece of baking parchment, from the short end. Leave to cool completely.
To decorate, unroll the cake and spread the cherry conserve over the surface. Whip the cream and spread it evenly on top, then roll up carefully and place on a serving plate. Sift a little icing sugar and cocoa powder on top, then serve.