Preheat the oven to 180C and line a small square baking tin with baking paper.
Break the chocolate in small chunks into a microwave bowl. Melt the chocolate in the microwave at 50% power for 2 minutes, stir it and leave to cool slightly.
In a larger bowl, mix the ripe avocado, sugar and a few drops of lemon juice. Using a mixer, slowly incorporate the beaten egg.
Keep mixing until you have a smooth mixture, then add the melted chocolate.
Sieve the flour and cocoa powder over the mixture and mix it well with a large spoon.
Finally mix in the chopped pistachios and pour the mixture into the baking tin.
Place in the center of the oven and cook for approximately 30-40 minutes or until a skewer comes out dry.
Cut into 6 squares and try not to eat them all at once!