Preheat the own 180 degrees Celsius. Process the beans for 2-3 minutes in a food processor until a smooth paste forms. Melt the butter in a pan and mix it in a bowl with the sugar and cocoa powder. Mix the other dry ingredients together in a second bowl. Mix the butter/bean mixture with the oat milk, whisk until well-combined. Add the dry ingredients little by little. The batter should resemble a thick pancake batter, add more flour or oat milk until the desired consistency is reached. In a small container mix the peanut butter with a bit of water until it is well-combined and can be folded into the batter. Chop the chocolate finely and add it at the end. Pour the batter into a well-greased form and bake for roughly 12-15 minutes or it the middle is firm and not runny. Let it cool for 20 minutes before cutting.
I actually made these brownies once before that they did not turn out as great as these ones, probably because there was no sugar in them. Lesson learned everything can taste good if you add sugar and butter.