Place the prunes in a bowl and pour over the brewed Assam tea. Cover and allow to infuse for 24 hours.
Preheat the oven to 170C(325F). Grease a 20 x 30 x 4 cm rectangular cake tin and line the base and sides with baking paper.
Sift the flour, cocoa, salt and baking powder in a bowl.
Put the chocolate, butter and sugar in a stainless steel bowl and sit over a pan of simmering water, making sure the base of the bowl does not touch the water. Stir for 10 minutes until the chocolate has melted. Allow to cool.
Add eggs one at a time to the chocolate mixture and mix with an electric mixer until well combined.
Add the flour, sour cream and mix with the mixer.
Stir in chocolate melts and prunes gently until just combined.
Spoon mixture into prepared tin and bake in the oven for 1 hour until just set.
Allow to cool completely in the tin before turning it out.
Dip knife in hot water, wipe down and cut to make neat squares.