Preheat the oven to 350°F. Lightly grease an 8 x 8-inch baking pan with oil or nonstick cooking spray and set aside.
Place the drained black beans in a food processor and process until smooth and creamy. Add the eggs, mashed banana, oil, sugar, cocoa powder, vanilla extract, baking powder, and salt and process until smooth. I have a small food processor, so I poured out the pureed black beans and mixed the other ingredients together by hand. Fold in the chocolate chips.
Pour the batter into the prepared pan, smooth the top with a rubber spatula. Top the batter with more chocolate chips, chopped walnuts, coconut, or cacao nibs.
Bake 30 to 35 minutes, or until the edges start to pull away from the sides of the pan and a toothpick inserted in the center comes out clean. Cool in the pan before slicing into 2-inch squares.