Grease and line a swiss roll tin with greaseproof paper.
In a large bowl over some simmering water, melt the butter and the chocolate then mix until smooth.
In a separate bowl, mix together the cocoa powder, flour, baking powder and sugar, then add this to the chocolate mixture and stir together well.
Beat the eggs and fold them in gently to achieve a silky consistency.
Pour the brownie mix into the baking tray and place it in the oven for around 25 minutes. You don't want to overcook them, so, unlike cakes, you don't want a skewer to come out all clean. The brownie mixture should just barely move when you shake the tray. They will come out slightly springy on the outside but still gooey in the middle.
Allow to cool in the tray, then carefully transfer to a large chopping board and cut into chunky squares.