Preheat oven to 350°F.

Grease a 9" cake pan and dust with cocoa powder. Line bottom of tin with a parchment round and grease the parchment.

Sift the cocoa powder into a bowl, add the baking powder, and whisk to combine.

Drain and rinse the black beans. Blend the beans, vanilla, salt, espresso powder if using, and 3 large eggs or 2 jumbo eggs in the blender until smooth.

Pour the bean mixture into a bowl and mix in the melted chocolate and the oil.

In another bowl, beat the remaining eggs and the sugar on high until light yellow in color and about doubled in volume.

Mix the cocoa powder into the bean mix.

Gently stir/fold the egg/sugar into the bean mix until combined.

Fold in any optional mix-in ingredients you're using (e.g. chocolate chips).

Pour into cake pan. Smooth top.

Bake for 30min.

Cool on rack in the tin for 10-15min, then turn out onto the rack and cool to room temperature.

The flavor develops and deepens a lot as it sits, so for best flavor let it sit overnight before eating. Heat pieces in the microwave before serving.