In a food processor or blender, add beans, 2 eggs and vanilla. Pulse several times until beans are broken up. Add 1/2 cup of cocoa powder and blend until smooth and no lumps remain.
In a large bowl, cream the butter, sugar and salt. Add remaining egg and mix well. Add bean mixture. Stir until well incorporated.
Grease an 8x8-inch square baking dish. Pour in batter, tapping lightly to remove any bubbles. Bake for 35 to 45 minutes, or until toothpick inserted into centre of brownie comes out clean.
. Allow brownie to cool to room temperature. In a large bowl add brownie and crumble. Chill for 1 hour.
Line a cookie sheet with parchment paper. Once brownie crumble is firm, remove from fridge and form into 1-inch balls. Place on cookie sheet and chill for 15 minutes.
In a large bowl, add remaining 1/4 cup of cocoa powder. One at a time, roll chilled truffles in cocoa powder, tapping off any excess. Serve on a platter or store in an airtight container, separating the layers with parchment.