Preheat the oven to 180C/350F/Gas 4 and grease and line two 20cm/8in sandwich tins with baking parchment.
Put the cocoa powder and boiling water into a large bowl and mix well to make a paste. Add the remaining ingredients and beat again until combined. This can also be done in a food processor, but take care not to over whisk. Divide the cake mixture between the prepared tins. Bake for about 25-30 minutes, or until well risen and shrinking away from the sides of the tin.
Meanwhile, for the icing and filling, measure the chocolate and cream together in a bowl and stand the bowl over a pan of simmering water for about 10 minutes, or until melted. Stir from time to time. Set aside and leave until cool and almost set.
Once baked, remove the cakes from the oven and allow to cool completely. Spread the tops of each cake with apricot jam. Fill the cakes with half of the icing and spread the remainder on top. Draw large “S” shapes over the cake with a palatte knife to give a swirl effect. Dust with icing sugar and serve in slices.