Preheat an oven to 300°F/150°C gas mark 2. Take an 8 inch /20 cm cake tin. Greased the base and line with baking parchment. Melt the chocolate for the cake by breaking it into a bowl and placing the bowl over a pan of simmering water. Cream the butter and sugar together until pale and fluffy. Beat in the egg yolks adding a little at a time. Remove the melted chocolate from the heat. Allow to cool. Fold gradually into the creamed butter mixture and add the vanilla extract. Sift the flour and baking powder together into a bowl. Return to the sieve and sprinkle into the butter and chocolate mixture, a little at a time. Fold in gently with a large metal spoon. Whisk the egg whites to the stiff-peak. Very carefully fold into the mixture, just a little at a time, bit by bit. Pour the mixture into the prepared cake tin, level the top and bake it on the middle shelf of the oven for about an hour, until firm and well risen. Allow to cool in the tin for about 10 minutes. Turn out onto a cooling rack. Allow to cool completely. Warm the apricot jam and brush the cake all over. Allow to cool. Pour the double cream into a bowl and place over a pan of simmering water. Break in the remaining 175g of chocolate and stir until melted. Add the glycerine and stir again. Pour the icing over the cake, making sure to cover the top and sides completely. Smooth. Allow to set for 2-3 hours. Serve.
