Preheat your oven to 350 F / 180 C / Gas Mark 4. Butter and line a twenty centimetre square baking tray ( the sides should be at least five centimetres deep ) with non-stick baking paper.

Break the dark chocolate into small pieces and place in a medium sized heavy-bottomed pan and add the butter and vanilla. Place on a very low heat until the butter and chocolate have melted, stirring occasionally to mix them together. Remove from the heat.

Sieve the plain flour, salt and cocoa powder into a medium bowl to remove any lumps. Save the sieve to one side.

Crack the eggs into a large bowl and add both sugars, whisk until thick and creamy. The mixture will be really pale and about double its original volume. This is called the ribbon stage. The whisk will leave a trail for a couple of seconds if removed from the mixture.

Pour in the chocolate mixture and gently fold together, in a figure of eight motion. Carefully mix whilst trying to keep in as much of the whisked air as possible.

Hold your sieve over the chocolate mixture and resift the cocoa, salt and flour mixture.

Gently fold this in using the same figure of eight action as before. Stop when the mix is sticky and thoroughly combined. Stir in the dark and milk chocolate drops.

Carefully transfer the brownie mixture into the lined baking tray and gently level out. Place into the oven and cook for twenty-five minutes. Pull the baking tray to the edge of the oven shelf and carefully shake the brownie. If the brownie is still wobbly in the centre quickly shut the oven door and cook for five more minutes

When the brownie is done, the sides should not be beginning to come away from the tin. Remove the oven and leave until completely cold. When cold remove from the tin and take off the baking paper.

Portion as required remember it is very rich. The brownie will keep in an airtight container for two weeks and can be frozen.