Make a ganache with the cream and dark chocolate coating. Whip the egg whites with the granulated sugar. Mix part of the whipped egg whites with the chocolate ganache, then add the sieved cornstarch and ground almonds mixture, and finally the rest of the whipped egg whites. Spread in a 3 cm-high 30 cm x 40 cm tin. Cook in a fan oven at 170°C (338°F) for about 15 min.
Bring the milk to the boil. Add the gelatine mix. Pour a quarter at a time onto the chopped dark chocolate coating. Gently mix the whipped cream into the chilled ganache using a rubber spatula. Set to one side.
Whip all of the ingredients together in a mixer.
Leave the biscuit in the baking tin. Spread the chocolate mousse on the biscuit and sprinkle with 300 g Morello cherries, then press them down lightly with a paddle. Place in the deep freeze. Cut out 10 cm by 2.5 cm rectangles and place on the edge of the chocolate slabs. Using a piping bag with a small St Honoré nozzle arrange the Chantilly on the top. Decorate with a cherry dipped in cooked sugar.