Preheat oven to 350F; grease an 8 by 8-inch baking pan with butter.
Whisk together the oil, egg, and brown sugar in a large bowl, then whisk in the banana, pumpkin, and vanilla. All at once add the flour, pumpkin pie spice, baking powder, salt, baking soda, and chocolate chips, and stir just to combine, being careful not to over-mix.
Pour the batter into the prepared pan and bake until golden and a toothpick inserted into the center comes out clean, about 30 minutes. Cool before cutting and serving.
Notes
Muffins: This recipe also makes 12 muffins; cook them at 350F until a toothpick inserted inside comes out clean, about 25 minutes.
Recipe by An Edible Mosaic™ at https://www.anediblemosaic.com/pumpkin-chocolate-chip-snack-cake/