Preheat oven to 350°F. Wrap the outside of a 9-inch springform pan tightly with two layers of heavy-duty foil to prevent leaks. Grease the inside with butter or nonstick spray. Place on a large rimmed baking sheet.

In a large bowl, whisk together mashed bananas, melted butter, sugar, egg, and vanilla until well combined. In a separate bowl, whisk flour, baking soda, cinnamon, and salt. Fold dry ingredients into wet until just combined - do not overmix. Spread evenly in the prepared pan and bake for 18-20 minutes until set on top but still slightly soft in the center. Leave oven on.

While the base bakes, beat cream cheese and sugar in a large bowl with an electric mixer on medium speed until completely smooth and fluffy, about 3 minutes, scraping down the bowl twice. Beat in sour cream and vanilla. Add eggs one at a time, beating just until combined after each. Do not overmix - this prevents cracks.

Pour cheesecake batter over the warm banana bread base, smoothing the top with an offset spatula. Return to oven and bake for 25 minutes until the edges are set but the center still jiggles slightly when you tap the pan.

While cheesecake bakes, make streusel: combine flour, brown sugar, and cinnamon in a small bowl. Cut in cold butter with a pastry cutter or your fingers until crumbly with pea-sized pieces. After the initial 25 minutes, quickly scatter streusel over the cheesecake and bake 10-12 minutes more until the topping is golden and the cheesecake center barely jiggles.

Turn off oven, crack the door open with a wooden spoon, and let cheesecake cool inside for 1 hour (this prevents cracking). Remove and cool completely at room temperature, then refrigerate at least 4 hours or overnight before releasing from pan and slicing. Run a thin knife around the edge before opening the springform.