Preheat oven to 350°F and grease a 13x9 inch baking pan.
In a large bowl, cream together the softened butter, granulated sugar, and light brown sugar until light and fluffy. Add in the oil and mix again.
½ cup unsalted butter, ¼ cup vegetable oil, 1 cup granulated sugar, ½ cup light brown sugar
Beat in the eggs, then add in the mashed bananas, vanilla extract, and sour cream.
2 large eggs, ¾ cup sour cream, 1 teaspoon vanilla extract, 1 & ½ cup bananas mashed
In a separate bowl, whisk together the flour, baking soda, baking powder, cinnamon, and salt.
2 cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon baking powder, ½ teaspoon salt, ½ teaspoon cinnamon
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
Pour the batter into the prepared baking pan and bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool completely in the pan on a wire rack.