Break the chocolate into pieces, set 50g of it aside and melt the rest in a bowl suspended over, but not touching, a pan of simmering water -or melt in the microwave.) As soon as the chocolate has melted remove it from the heat. Chop the remaining 50g into the size of small stones.
Beat the sugar and butter using electric beaters for several minutes until white and fluffy.
Break the eggs into a small bowl and beat them lightly with a fork. In another bowl, sift together the flour, cocoa and baking powder and mix in a pinch of salt.
Add the beaten egg to the butter/sugar mix a little at a time beating well between additions.
Mix in the melted and the chopped chocolate with a large metal spoon.
Lastly, fold in the flour and cocoa, gently and firmly, without knocking any of the air out. Scrape the mixture into the prepared cake tin, smooth the top and bake for 30 minutes.
The top will have risen slightly and the cake will appear slightly softer in the middle than around the edges. Pierce the centre of the cake with a fork - it should come out sticky, but not with raw mixture attached to it. If it does, then return the brownie to the oven for three more minutes. It is worth remembering that it will solidify a little on cooling, so if it appears a little wet, don't worry.