In a large mixing bowl, thoroughly mash 1 1/2 cups of black beans. If you have a food processor, that’s even better. You want the end result to be smooth.
Whisk together the 6 egg yolks and add that to the mixture.
Mix in the tablespoon of vanilla, 1/4 cup finely ground coffee, 1/4 teaspoon sea salt, and 1/2 cup of sugar.
Melt the 1/2 cup of butter over a medium-low heat.
Once the butter is melted, turn off the heat but leaving the pot on the burner, and add in the chopped chocolate. Stir until the chocolate has completely melted and combined with the butter.
Slowly add the melted chocolate to the bean mixture, stirring vigorously. You don’t want the heat from the butter-chocolate mixture to cook the egg yolks. So it is key to add a little at a time for this reason.
Beat the egg whites in a separate bowl until stiff. Fold them into the brownie batter until completely combined.
Transfer the mixture to a small baking dish and bake for about 30 minutes at 325° F. Test with a toothpick at the 20 minute mark and again at 30 for completeness.
Remove the brownies from the oven, let cool. Cut into nine individual squares and dust with powdered sugar.