Prepare the soufflé dish: Cut a piece of foil or parchment, longer than the circumference of the soufflé dish. Double it for stiffness. Wrap around the outside rim so that it rises at least an inch above the top of the dish, and tape to secure.
Purée the berries with 1/4/55 g of the sugar, the lemon juice and salt. Strain and reserve.
Put the yolks in a non-reactive bowl with 1/4 cup sugar/55 g and 1/2 cup/125 of the berry purée. (Reserve the remaining for another use.)Set over a water bath, making sure the bowl does not touch the simmering water below, and whisk until the sugar dissolves and it is warm in the centre. Remove the bowl from the saucepan and set aside.
Put 1/4 cup/60 ml of water into a ramekin, sprinkle over the gelatine, and let soften 5 minutes. Set the ramekin in the hot water in the saucepan and let it melt, about 2 minutes. It should be translucent. Whisk this into the berry mixture, and let the whole cool until it thickens slightly. Don’t let it set too far.
Beat the whites with the cream of tartar until foamy. Gradually incorporate the remaining 1/4 cup/55 g sugar, and beat to thick, shiny peaks like whipped cream. Whip the cream in a separate bowl. Stir a spoonful of the whites into the berry mixture to lighten it. Scrape the remaining whites and cream on top and fold the whole together.
Spoon into a 6-cup/1.5 litre mould, cover, and refrigerate until firm, about four hours. To serve, remove the collar and spoon into dishes. This is best eaten on the day it is made or the next day. After that, the gelatine starts to break down.