Sift together the flour, bicarbonate of soda and cocoa powder. Cream together the butter and half the sugar until soft and fluffy. Gradually whisk in the eggs, then whisk in the rest of the sugar. Mix the milk with the lemon juice to sour it, then fold into the flour mixture until smoothly combined.
Grease 2 cake tins, 20 cm (8 inch) in diameter, and line the bases with nonstick baking paper. Spoon the mixture equally into the prepared tins and level over the surface of both.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 30 minutes until risen, springy to the touch and shrinking away from the edges of the tin. Cool in the tins for 10 minutes, then upturn the cakes on to a wire rack to cool.
Melt the dark and milk chocolate together in a bowl over a pan of simmering water (see
Slice each cake in half horizontally to make 4 layers. Put one layer on the serving dish and spread with one-quarter of the frosting. Top with another cake layer, then some more frosting. Continue layering cake and frosting, ending with frosting on top.