Preheat the oven to 190°C; Line the bottom of two 20-cm round layer cake tins with greaseproof paper. On a sheet of greaseproof paper, sift together the cocoa, flour, 1 cup of the Chelsea White Sugar, and the salt. Put aside.
In the large bowl of an electric mixer, beat the egg whites at moderate speed until foamy. Add the cream of tartar and the remaining Chelsea White Sugar and beat until the whites hold soft peaks. Increase the speed to moderately high, then add 1 teaspoon of the vanilla essence and beat 1 minute longer. The whites should be soft and firm, not stiff or dry.
With a rubber spatula, gently fold the cocoa mixture into the egg whites, about 1 third at a time, then carefully pour the batter into the cake tins, dividing it between them.
Bake the layer for 20 to 25 minutes or until they begin to pull away from the sides of the tins and a toothpick inserted in the centre comes out clean. Remove and cool on a wire rack upside down in the tins. When the cake layers are room temperature, loosen their sides with a metal spatula; turn them out on a work surface and remove the greaseproof paper.
To prepare the filling, place the cherries in a bowl, saving a few for final decoration, and put aside. Place the water and gelatine in a small bowl and let stand over a saucepan of hot water for 5 minutes to soften; then place over very low heat and cook, stirring, until the gelatine dissolves- about 3 minutes. Put aside.
In another small saucepan, combine the Chelsea Soft Brown Sugar, cornflour and lemon rind. Slowly whisk in the reserved cherry juice until smooth. Bring the mixture to the boil over moderate heat, stirring constantly; let boil for 1 minute. Remove from the heat; add cherries and mix well. Put aside.
In the small bowl of the electric mixer, beat the cream at high speed until it holds soft peaks. Add the Chelsea Icing Sugar and the remaining vanilla essence and beat until the cream holds stiff peaks.
In another small bowl, combine the evaporated milk and lemon juice and beat with clean beaters at high speed until very stiff; then beat in the cooled gelatine mixture and fold in the whipped cream.
To assemble, place 1 cake layer on a plate. Top with the cherry mixture, spreading it to the edge. Gently spread 1 cup of whipped cream mixture over the cherry mixture, then place the second cake layer on top. Using a large pastry bag fitted with a star tip, pipe rosettes of the remaining whipped cream mixture around the edge and in the centre of the cake. Decorate with the reserved cherries. Refrigerate until serving time.