Preheat the oven to 180C/350F/Gas 4 (160C fan).
Put the cocoa in a large bowl, add the boiling water and mix until smooth. Add the baking spread or butter and beat into the cocoa until smooth, then add the remaining cake ingredients and mix until combined â€“ this can be done in a mixer or by hand.
Pour into the tin, spread evenly and bake in the preheated oven for about 30-35 minutes, or until well risen, just firm to the touch and shrinking away from the sides of the tin. Leave to cool in the tin for 10 minutes, then turn out and peel off the baking parchment. Brush the top of the cake with apricot jam and leave to cool completely
For the icing, warm the cream in a small pan until hot, then add the milk chocolate and stir until completely melted and smooth. Leave to cool a little so that it is a thick pouring consistency (this wonâ€™t take long depending on room temperature). Pour the icing over the cake, spreading out with a palette knife to cover the cake completely.
To decorate, melt the white chocolate in a small bowl over a pan of gently simmering water. Place in a small piping bag, snip the end and pipe in lines down the cake, about 1cm/Â½in apart. Using a cocktail stick, drag through the white chocolate in opposite directions across the cake to give a feathered effect.