Get about 150 grams (about 3/4 cup) of dried adzuki beans. You may have a bit of leftover. Rinse them in water to get rid of any impurities
If you are going to cook this on the stove, soak the beans for at least 4 hours or overnight. Discard the soaking liquid. Prepare a big pot and add water, about 2-inches above the beans. Bring the water to a boil and then lower to medium heat and boil the beans until they are soft and you can easily mash it with your fingers, about 1 to 1 1/2 hours or so. You may need to top off with some water along the way
If you plan to cook with pressure cooker like Instant Pot, you don't need to soak the beans. Place the beans in the insert of the instant pot. Add water about 1 inch above the beans. Cover the lid. Turn the steam release handle to seal. Press pressure cooker and set timer to 45 minutes if you did not soak the beans. If you choose to soak the beans, set the timer to 25 minutes
Once the beans are cooked, drain off the cooking liquid
Prepare the brownie batter:
Preheat the oven to 350 F (180 C) for conventional oven, 330 F (165 C) for convection oven
Line your 8 x 8 square pan with parchment or aluminum foil (sprayed with non-stick spray or brush with some oil to prevent sticking). You don't need to grease the parchment if that's what you use. You can grease the pan and then line with parchment so the paper won't move around when you spread the brownie batter on it
You will need a food processor to prepare the brownie batter. Place the cooked beans and the rest of the ingredients (except for the cacao nibs) in the bowl of the food processor. I include the chocolate chips in there and they will be blended with the rest of the ingredients. Process until you get a smooth consistency. It's going to be a thick batter that looks like a mud at this stage.
Spread the batter evenly on the prepared pan using the back of the spoon or spatula
Sprinkle with some cacao nibs or more chocolate chips if you choose to use
Baking:
Bake on the middle rack in the preheated oven for 13-15 minutes. The thin edges look set but the rest to the center are still very soft to the touch and not set yet. It is very very important not to overbake brownies. If the whole thing look set and dry, you won't have that fudgy texture
Cooling:
Let the pan sit on a cooling rack and just let it cool down completely in the pan. Don't try to cut into a warm brownie, it won't hold up
Chill in the fridge:
Now comes the hard part, you cannot eat this yet! I mean, you can, but I strongly recommend you not to, not yet! Once the brownies have cooled down completely, lightly cover it with a cling wrap and put this in the fridge overnight
You will be rewarded with that moist, dense, and fudgy texture. The refrigeration helps to set the brownies further and I absolutely love a cold fudgy brownies :)
To serve:
You can serve it cold or if you prefer it warm, you can warm it up for a few seconds in the microwave and serve it as is or with some ice cream if you want to indulge a bit (since the brownies are healthier, I won't tell!).
I recommend not telling anyone that there are beans in the brownies yet when you serve them . Just see how they react when they eat it :) You'll be surprised!
To store:
Keep them in an air-tight container and they can be kept for 2-3 days in the fridge. They freeze very well too. Simply thaw at room temperature or thaw overnight in the fridge before serving