Preheat the oven to 190°C (Gas Mark 5 / 375°F).
Break the chocolate into squares and place in a bowl. Microwave for a minute or two until melted, then set aside to cool a little.
Meanwhile, beat the eggs with the light brown sugar in a large bowl. Add the melted chocolate, Fine Dark Cocoa Powder, ground almonds, Madagascan Vanilla Extract, and Dr. Oetker Bicarbonate of Soda, and mix well.
In a separate bowl, thoroughly mash the avocado flesh, and add it to the chocolate mixture. Mix thoroughly.
Lightly grease and line a deep metal baking tray (measuring approximately 8 x 8 inches) with baking paper. Transfer the brownie mix to the tray, and spread it out evenly. Bake for around 35 minutes, until a cocktail stick comes out clean. Leave to cool in the pan before cutting and serving.