Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper.
In a large mixing bowl, combine the banana cake mix, mashed bananas, softened butter, egg, and vanilla extract.
Mix the ingredients together using a hand mixer or wooden spoon until a smooth dough forms.
Using a tablespoon or cookie scoop, drop rounded spoonfuls of dough onto the prepared baking sheets, spacing them about 2 inches apart.
Bake in the preheated oven for 10 to 12 minutes, or until the edges are lightly golden and the centers are set.
Remove the cookies from the oven and allow them to cool on the baking sheets for 5 minutes.
Transfer the cookies to a wire rack to cool completely before serving.