01 - Preheat oven to 350°F. Grease and flour a 9-inch round or square cake pan, tapping out any excess flour.
02 - In a large mixing bowl, beat the mashed bananas, vegetable oil, granulated sugar, eggs, and vanilla extract together until the mixture is smooth and well blended.
03 - In a separate bowl, whisk together the all-purpose flour, baking soda, baking powder, salt, and ground cinnamon if using, until evenly distributed.
04 - Gradually add the dry ingredient mixture to the wet ingredients, folding gently with a spatula until just combined. Be careful not to overmix, as this will make the cake dense.
05 - If desired, gently fold in the chopped walnuts, pecans, or chocolate chips until evenly distributed throughout the batter.
06 - Pour the batter into the prepared cake pan and use a spatula to smooth the surface evenly.
07 - Bake on the center rack for 35 to 40 minutes, or until a toothpick inserted into the center of the cake comes out clean and the top is golden brown.
08 - Allow the cake to cool in the pan for 10 minutes, then carefully invert it onto a wire rack to cool completely before serving or frosting.