Place an 8-inch by 8-inch square of parchment paper into the bottom of a pan of the same size.

Stir together the oats, crispy rice cereal, walnuts, and banana chunks in a large mixing bowl. Set aside.

In a medium saucepan, melt the butter then stir in the brown sugar, honey, and peanut or sunflower seed butter. Bring to a boil. Once it's bubbling hard, let it boil for 2 minutes. Remove the pan from the heat and stir in the vanilla. Be careful, it will bubble up crazily when you add the vanilla!

Immediately pour over the dry ingredients and gently but quickly stir it in thoroughly. Turn it into the parchment lined pan right away and press into the pan evenly. Sprinkle the chocolate chunks or chocolate chips over the pan and let stand, undisturbed, until the chocolate looks shiny, about 5 minutes. When the chocolate looks very shiny it should all be melted and can be spread evenly with an offset or silicone/rubber spatula. Let stand until the chocolate has firmed back up.

Run a knife around the edge of the pan to loosen any chocolate that stuck to it, then turn the large block of granola bar onto a cutting board. Flip it chocolate side up and cut into bars of your desired size. Store in an airtight container at room temperature for up to a week... not that they'll ever last that long!