Preheat oven to 190°C (375°F/Gas 5). Line the bases of two 20 cm (8 inch) cake tins with baking (parchment) paper.
Onto a plate, sift together the cocoa, flour, 1 cup sugar and the salt.
In a large bowl using an electric mixer, beat the egg whites on medium speed until foamy. Add the cream of tartar and remaining sugar and beat until the whites hold soft peaks. Increase the speed to medium–high, then add 1 teaspoon vanilla and beat for a further minute. The whites should be soft and firm, not stiff and dry.
With a spatula, gently fold the cocoa mixture into the egg whites, one-third at a time, then carefully divide the mixture between the tins. Bake for 20–25 minutes, or until the mixture begins to pull away from the sides of the tins and a skewer inserted in the centres comes out clean.
Remove from the oven. Invert onto a wire rack and allow to cool upside down in the tins. When the cakes are at room temperature, loosen their sides with a metal spatula. Turn them out on a work surface and remove the paper.
To prepare the filling, put the water and gelatin in a small bowl and let stand over a saucepan of hot water for 5 minutes to soften. Place over very low heat and cook, stirring, for about 3 minutes, or until the gelatin dissolves. Set aside.
In another small saucepan, combine the brown sugar, cornflour and lemon zest. Slowly whisk in ½ cup (125 ml) of the reserved cherry juice until smooth. Bring the mixture to a boil over medium heat, then add most of the cherries (reserving some for decoration) and mix well. Set aside.
In a small bowl using an electric mixer, beat the cream on high speed until it holds soft peaks. Add the icing sugar and the remaining vanilla and beat until the cream holds stiff peaks.
In another small bowl, combine the evaporated milk and lemon juice and beat with clean beaters on high speed until very stiff. Beat in the cooled gelatin mixture and fold in the whipped cream.
To assemble, place a cake layer on a cake plate. Top with the cherry mixture, spreading it to the edge. Gently spread 1 cup whipped cream mixture over the cherry mixture. Place the second cake layer on the top. Using a large piping (icing) bag fitted with a star tip, pipe rosettes of the remaining cream mixture around the edge and in the centre of the cake. Decorate with the reserved cherries. Refrigerate until ready to serve.