Preheat the oven to 170C/325F/Gas3. Line a 30x20x3.5cm/12x8x1½in tin with lightly buttered greaseproof paper or foil.
Put the plain chocolate and butter in a large bowl, place over a pan of simmering water and allow to melt.
Sieve the flour and baking powder into a bowl and set aside. Remove the melted chocolate from the heat and stir in the sugar. Add the eggs and vanilla essence. Fold in the flour, nuts and chocolate.
Pour the chocolate mixture into the prepared cake tin. Place in the oven and bake for 20-25 minutes. The top should be firm but the inside should feel soft when cooked. Allow to cool in the tin. Remove the brownies from the tin and cut into squares.