Preheat oven to 350˚F, and butter a 8x8" brownie pan.
In a processor, blend beans and sugar.
Melt butter with chocolate in the microwave.
Mix together the bean and chocolate mixtures. Add the eggs, vanilla extract, and salt. Add espresso powder, chocolate chips, and/or walnuts as desired.
Bake at 350˚F for ~30 minutes, until brownies slightly crack. A knife may not come out clean. Importantly, let the brownies cool for a bit before cutting. As usual with brownies (especially bean brownies), these taste better the next day.