For the chocolate icing
1. Preheat the oven to gas 4, 180ºC, fan 160ºC. Line a rectangular tin (27cm x 17.5cm x 3.5cm) with non-stick baking paper. Chop 175g of the chocolate for the brownies and put into a small heavy-based pan with the butter. Melt over a low heat, stirring occasionally. Remove from the heat and leave to cool.
2. Chop 50g chocolate into small chunks and set aside. When the chocolate has cooled, whisk the eggs and sugar together in a large bowl using an electric hand mixer, until thick, pale, fluffy and almost doubled in volume. Pour in the cooled chocolate, then gently fold in. Sift over the flour, baking powder and cocoa powder and gently fold in. Carefully stir in the chopped chocolate, without over-mixing.
3. Pour the mixture into the lined tin and lightly spread it into all the corners. Bake for 25-30 mins, or until the middle is just set and the top is slightly crusty. (If still wobbly in the centre, leave a bit longer.) Remove and leave until completely cold before removing from the tin.
4. Very finely chop the chocolate for the icing. Put into a small heavy-based pan with the cream and sugar. Melt over a low heat very briefly, just until the chocolate starts to melt, removing it before it boils. It will be quite thick, so pour in 1-1½ tbsp of boiling water to thin it to a drizzling consistency. Drizzle over the brownies then, when set, cut into 15 squares.
Tip: Leave the brownies in the tin until completely cold, as they'll crack if turned out too early.