In a medium bowl, mix together the flour, sugar, butter and cinnamon with your fingertips until the mixture forms small crumbs; set aside.

Preheat the oven to 350º F. Spray an 8 x 8-inch baking pan with non-stick baking spray and line with a sheet of parchment paper leaving a generous overhang for easy removal; set aside.

In a medium bowl whisk together the flour, baking powder, baking soda, and salt; set aside. In a separate bowl stir together the mashed bananas, milk, vanilla and vinegar; set aside.

In the bowl of a stand mixer fitted with the paddle attachment (or a hand-held mixer), cream together the butter and sugar on medium-low speed until light and fluffy, about 2-3 minutes. Add the egg and beat until incorporated, scraping down the sides of the bowl. Add the banana mixture (it will look curdled) and beat until combined. Reduce the mixer to the lowest speed, gradually add the flour mixture, mixing just until combined. Spoon the batter into the prepared pan and smooth the surface with an off-set spatula. Sprinkle the streusel evenly over the batter.

Bake until the cake springs back when gently pressed in the center and a toothpick inserted in the center comes out clean, about 35 minutes. Transfer to a wire rack and cool for 10 minutes. Using the parchment overhang, remove the still warm cake and place on the rack to cool completely.

Store, wrapped in plastic or in an airtight container for up to 3 days or freeze for up to 2 weeks. Thaw at room temperature for about 3 hours before serving. Enjoy!