200 g dark chocolate (minimum 75% cocoa), cut in small pieces + 200 g unsalted butter, diced
180 g sugar + 3 big eggs, room temperature + 1 tsp vanilla essence
80 g flour, sieved
50 g walnuts, cut in small pieces
1 avocado + 1 tbsp lemon juice (fresh) + 3-4 tbsp confectioner's sugar
Heat the oven at 180 degrees Celsius. Prepare a cake ... with butter and baking paper (20x20cm).
Melt the chocolate over bain-marie, mixing from time to time. When chocolate is liquid, remove from heat and add the butter, mixing untill it melts, too. Don't let the chocolate to be in direct contact with the hot water vapors.
In another bowl, mix sugar, vanilla and eggs, untill you get a fluffy cream. Add the flour, little at a time and mix. Add the walnuts and mix. Add the chocolate mixture and mix with a silicon spatula. Pour the batter in the tray and bake it in the oven for 20 minutes.
In the mean time prepare the frosting. Take the avocado, ripe, but not brown, mash it, then add the lemon juice and mix it well. Start adding the sugar, little at a a time, mixing well. I did not use to much sugar, as I'm a big fan of not too sweet frostings, but feel free to adapt it to your taste.
Take the brownies out of the oven and force cool the cake: put the hot tray in a bigger tray with cold water and ice cubes. Let it cool down this way, then cut it.
Serve the brownies with the avocado frosting. Keep both the brownies and the frosting in refrigerator. I love cold brownies... 
