Preheat oven to 350°F (175°C). Grease and flour an 8-inch square baking pan. If you like, line the pan with parchment paper and grease the paper.
Whisk together the flour, oats, baking soda, and salt. Set aside.
Using an electric mixer on medium speed, beat the butter and brown sugar until light and fluffy. Add the eggs, one at a time, mixing well after each addition. Mix in the bananas and vanilla.
Reduce mixer speed to low. Gradually add the flour mixture, mixing until combined.
Transfer the cake batter to the prepared pan.
To make the crumb topping:
Mix together the oats, brown sugar, pecans, butter, and cinnamon until crumbly. Sprinkle the mixture evenly over the cake batter.
Bake 35-40 minutes, or until a pick inserted into the center of the cake comes out clean.
Cool in the pan on a wire rack for 20-30 minutes before serving.
Notes
Recipe adapted from Buttercup Bakes at Home.
Store in an airtight container at room temperature up to 4 days.