Preheat the oven to 350°F. Line an 8- or 9-inch round cake pan with parchment paper (or use a silicone cake pan).
Combine the beans, date syrup, and cocoa powder in a food processor; process just until smooth. Add the oats, baking powder, and baking soda; process just until combined.
Spread batter into the prepared cake pan. Sprinkle with pistachios and chocolate pieces (if using). Bake for 35 minutes. Cool on a wire rack. Cut into wedges.
Links to ingredients for Outrageously Healthy Brownies