Preheat the oven to 320°F/160°C.
Line a baking pan with parchment paper and grease it with oil. Drain the black beans and rinse thoroughly in a colander under running water for 2 minutes. Put the beans in the food processor with the eggs and sugar and mix nonstop until a completely smooth mixture is formed (about 2 minutes). Add vanilla, oil, cocoa powder, baking powder, baking soda, coffee *optional*, and salt.
Process until everything is mixed well.
Pour the batter into the baking pan.
Bake for 30-35 minutes until the cake is puffed, burst at the top, and slightly gooey in the center. You can insert a cake tester/skewer into the center of the cake; if it comes out clean with a few small crumbs, the cake is ready.
Let it cool completely before cutting into squares.
*Optional* dust with powdered sugar.