- Preheat the oven to 190C / fan 170C
- Grease a 24cm square tin with a little butter. Cut a piece of greaseproof paper the same size as the bottom of the tin and place it in the bottom of the tin - this wii help you get the brownies out.
- Place the chocolate and butter in a large bowl set over a pan of simmering water. Heat gently. Don't let the water boil rapidly and touch the bottom of the bowl. Heat and stir until the chocolate and butter is melted.
- Remove from the heat and add the sugar. Stir until well combined.
- Leave to cool for 5 minutes. Then add the eggs and egg yolk one by one, beating well after each egg. You should have a dark glossy mixture.
- Add the ground almonds, salt, flour and baking powder and using a metal spoon fold gently, in a figure of eight movement. Do not over mix. Finally fold in the pistachio nuts.
- Pour the mix into the tin and bake for 20-25 minutes. It should be set, but with a little 'gooeyness' in the middle. A good way of testing is to push a skewer into the middle. It should come out fairly clean but with a few moist crumbs sticking to it.
- Leave in the tin to cool, then cut into squares and dust with icing sugar.