Preheat the oven to 175°C (350°F) with a rack in the centre. Line a 20 cm (8 inch) square tin with parchment, leaving overhang on two sides.
Rinse the black beans very well and drain them thoroughly; any canning liquid gives a bean taste.
Add the beans, eggs, cocoa, protein powder, maple syrup, oil, vanilla, baking powder and salt to a food processor or blender and blend for 1 to 2 minutes, scraping down, until completely smooth.
Stir in half the chopped chocolate, spread the batter in the tin and scatter the rest of the chocolate on top.
Bake for 18 to 20 minutes, until the top looks set and a toothpick in the centre comes out with a few moist crumbs. Do not overbake or they turn dry.
Cool completely in the tin, at least 1 hour, then chill 30 minutes for clean cuts. Lift out and cut into 12 squares.
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Tips
Rinse the beans very well; that is what keeps them from tasting beany.