1. Preheat your oven to 350°F and grease your 9×13-inch baking pan generously. Line the bottom with parchment paper.

2. In a medium bowl, whisk together flour, brown sugar, granulated sugar, cinnamon, and salt for streusel. Cut in cold cubed butter with a pastry cutter until large crumbs form. Refrigerate while making cake.

3. In your large mixing bowl, mash bananas until mostly smooth. Stir in melted butter, then add both sugars, eggs, and vanilla. Whisk vigorously for 45 seconds until smooth.

4. Stir in sour cream until completely incorporated. The mixture may look slightly curdled but that’s normal.

5. Whisk together baking soda, baking powder, and salt, then sprinkle over wet mixture and stir thoroughly. Add all the flour and gently fold just until no dry streaks remain.

6. Pour batter into prepared pan and spread evenly, pushing into corners. Use back of spoon to smooth the top.

7. Remove streusel from fridge and sprinkle all of it evenly over the batter. Gently press down slightly so it adheres.

8. Bake for 35-40 minutes, until streusel is deeply golden and a toothpick inserted in center comes out with just a few moist crumbs.

9. Cool in the pan on a cooling rack for at least 20 minutes before slicing and serving.