Pre-heat oven to 350 degrees Fahrenheit. Line a 9×9 pan with parchment.
In the bowl of a stand mixer, beat the butter, sugar and vanilla on medium speed for about 2 minutes.
Add the egg and mashed banana and continue to beat for an additional 3 minutes. The mixture should look wet and lumpy. The butter will be in small chunks but not combined with the rest of the ingredients.
In a medium bowl, combine the rest of the ingredients and whisk them well.
Add the wet ingredients to the dry, folding together until just combined.
Add the chocolate chips.
The dough will be a bit sticky and hard to spread. Use a spatula to spread it into the pan as evenly as possible.
Place the pan in the oven on the middle rack and bake for 20 minutes or until a toothpick comes out mostly clean.
Allow them to cool on a cooling rack. Then cut into squares and enjoy!