Traditional Chocolate Cake

Preheat a 180C oven.
Line 2 x 8-9 in (20-23 cm) round cake tins with greaseproof paper.
In a heatproof bowl set over a pan of simmering water, or in a doubleboiler, combine the chocolate, one-third of the milk, the brown sugar and egg yolk.
Cook, stirring, until smooth and thickened.
Let cool.
Sift the flour, bicarbonate of soda and salt into a bowl.
Set aside.
With an electric mixer, cream the butter or margarine with the caster sugar until light and fluffy.
Beat in the whole eggs, one at a time.
Mix in the vanilla.
On low speed, beat the flour mixture into the butter mixture alternately with the remaining milk, beginning and ending with flour.
Pour in the chocolate mixture and mix until just combined.
Divide the cake mixture evenly between the cake tins.
Bake until a skewer inserted in the centre comes out clean, 35-40 minutes.
Let cool in the tins on wire racks for 10 minutes, then unmould the cakes from the tins onto the wire racks and let cool completely.
For the icing, melt the chocolate in a heatproof bowl set over a pan of hot, not boiling, water, or in the top of a double boiler.
Remove the bowl from the heat and stir in the the salt and soured cream.
Let cool slightly.
Set 1 cake layer on a serving plate and spread with one-third of the icing.
Place the second cake layer on top.
Spread the remaining icing all over the top and sides of the cake, swirling it to make a decorative finish.