Heat the oven to 180°C/350°F, or 160°C fan. Grease and line three 20 cm cake pans.
Sift the cocoa and flour. Beat the eggs for 30 seconds, slowly add the sponge sugar over 45 seconds, add vanilla, and beat about 7 minutes until pale and tripled in volume.
Fold in the dry mixture gently, then fold in the cooled butter. Divide the thin batter among the pans. Bake for about 25 minutes, until springy and clean when tested. Cool 10 minutes in the pans, then cool completely on a rack.
Drain the cherries and reserve the juice. Mix 1/4 cup juice with the cornflour. Simmer another 1/3 cup juice with the syrup sugar, stir in the slurry, simmer 1 minute, add kirsch if using, and pour over the cherries. Cool completely.
Melt the chocolate in three 20-second microwave bursts. Spread it thinly on a tray, chill briefly, then scrape curls or shavings. Refrigerate them.
Whip the cream with icing sugar and vanilla to firm peaks. Keep it chilled.
Place one cake layer upside down on a platter. Brush with 1/4 cup syrup, spread with 1 cup cream, and add half the cherries. Repeat with the second layer, then add the third.
Cover the cake with cream, reserving 1 1/2 cups for piping. Pipe the top, add decorating cherries, finish with chocolate curls or shavings, and chill at least 4 hours or overnight. Remove 30 minutes before serving.