Preheat the oven to 180ºC (350ºF), grease a round springform cake pan and line with baking paper.
Melt the chocolate in a heatproof bowl.
Cream the butter and sugar in a bowl until pale and fluffy, add the egg yolks one at a time, beating after each addition.
Beat in the melted chocolate, sift the flour over the mixture and fold in evenly.
Whisk the egg whites until stiff, then stir a quarter into the chocolate mixture to lighten it.
Fold in the remaining whites, turn the mixture into the cake pan and smooth it level.
Bake for 50 minutes or until firm, turn out on to a wire rack to cool.
Using a small saucepan heat the apricot jam with the lemon juice until melted, then sieve.
Slice the cake horizontally into two equal layers.
Brush the cut surfaces and sides of each layer with the apricot glaze, then sandwich together, placing the jam-covered surfaces against each other. Place on a wire rack.
Mix the icing ingredients in a heavy pan.
Heat gently, stirring until thick, simmer for 3 to 4 minutes without stirring, until the mixture registers 95ºC (200ºF) on a sugar thermometer.
Pour quickly over the cake and spread evenly, leave to set.