Prep the pan. Heat the oven to 350°F (175°C). Line an 8-inch square baking pan with parchment paper, letting it overhang for easy lifting. Lightly grease the sides.

Mash the bananas. In a large mixing bowl, mash the bananas until mostly smooth with just a few small lumps remaining.

Whisk in the wet ingredients. Add the nut butter, maple syrup or honey, egg (or flax egg), and vanilla. Whisk until the mixture looks glossy and unified.

Stir in the dry ingredients. Sprinkle in the cocoa powder, oat flour or whole-wheat flour, baking soda, and salt. Fold gently with a spatula until no dry streaks remain. Do not overmix.

Add chocolate. Fold in the chocolate chips or chunks and any optional add-ins. The batter will be thick and fudgy.

Spread and smooth. Transfer the batter to the prepared pan and smooth the top with a spatula. For extra chocolate, scatter a few chips on top.

Bake. Bake for 20–25 minutes, until the edges are set and the center looks slightly underbaked but not wet. A toothpick inserted near the center should come out with moist crumbs, not batter.

Cool completely. Let the brownies cool in the pan on a rack for at least 30–45 minutes. For the cleanest slices and best fudgy texture, chill for 30 minutes before cutting.

Slice and serve. Lift out using the parchment overhang, slice into 12–16 squares, and enjoy.