Drain the cherries into a sieve and reserve the syrup. Line and grease a deep 9 inch round cake tin. Put the butter in a bowl over a pan of warm water, beat until very soft.
Whisk the eggs and sugar together in a large bowl standing over a pan of simmering water until pale and creamy, and thick enough to leave a trail on the surface when the whisk is lifted. Remove from the heat and whisk until cool.
Sift the flour and cocoa together and then lightly fold into the egg mixture. Fold in vanilla essence and softened butter.
Turn the mixture into the prepared tin and tilt to spread evenly. Bake at 350F (150c) mark 4 for about 35-40 minutes until well risen, firm to touch and beginning to shrink away from the sides of the pan. Turn out onto a wire rack and leave to cool.
Cut the cake into three layers. Place one layer on a flat plate. Sprinkle about 3 tablespoons of cherry syrup over the cake. Spread a thin layer of cream over the soaked cake. Reserve a third of the cherries for decoration, scatter half the remainder over the cream.
Repeat layers of sponge, syrup, cream and cherries. Top with the third cake round and spoon over the remaining syrup.
Spread a thin layer of cream around the side of the cake. Press on the chocolate caraque, reserving some. Pipe whirls of cream around the edge of the cake and top each with a chocolate caraque.
Fill the centre with the reserved cherries. Blend the arrowroot with 3 tbsp cherry syrup. Place in a small pan; bring to the boil, stirring, for a few minutes until the mixture is thickened and clear. Brush the glaze over the cherries.
To make the chocolate caraque, melt the chocolate in a heatproof bowl standing over a pan of simmering water. Remove from the heat and spread out thinly on a marble slab or clean work surface. Leave to set until no longer sticky to touch.
Hold a large knife with both hands; push the blade across the surface to shave off long chocolate curls.