Preheat & Prep. To start, preheat oven to 350 degrees F. Grease a 9-inch cake pan or 9-inch springform pan. (I recommend the springform pan, if you have it!)

Cream butter & Sugar. In a large bowl, combine butter and brown sugar. Beat on medium speed with a hand mixer 2-3 minutes, till light and fluffy.

Finish Wet Ingredients. Add mashed bananas and stir to combine. Add eggs, one at a time, then the vanilla extract.

Add Dry Ingredients. Scrape down the sides of the bowl, then add flour, baking soda, and salt. Mix to combine, then fold in the oats. Pour cake batter into the prepared pan.

Make Topping. In a small mixing bowl, whisk oats, brown sugar, and flour. Add the butter and mix until a crumbly mixture forms. Sprinkle over cake.

Bake at 350 degrees for 30-35 minutes or until a toothpick inserted in the center of the cake comes out clean. Run knife around the outside of the cake to loosen it from the sides as it cools.

Serve & Store. Let cake cool, then cut into slices & serve. Store leftover cake in an airtight container at room temperature 2-3 days, or freeze up to 2 months.