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