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Heat oven to 160C/140C fan/gas 3. Line a 22 x 22cm square tin with baking parchment. Put the chocolate, butter and sugar into a large pan with 100ml hot water and gently melt together. Set aside for 2 mins, then stir in the soured cream followed by the eggs. Finally, stir or whisk in the flour and cocoa until lump-free, then pour into the prepared tin. Bake for 50-55 mins until a skewer comes out clean. Sit the tin on a wire rack to cool.
Meanwhile, make the icing. Gently melt together icing ingredients in a heatproof bowl set over a pan of barely simmering water, then chill or cool until spreadable.
To decorate, carefully turn out the cooled cake and peel off the baking parchment. Spread the icing over the top and scatter with hundreds and thousands, then cut the cake into triangles or fingers before serving.