Prepare a 9-inch springform pan: line the bottom with parchment, butter and flour the parchment and pan sides.
In a the bowl of a stand mixer fitted with the paddle attachment, blend together the flour, sugar, baking soda, baking powder, and salt, and set aside.
In the bowl of a food processor, blitz the banana and sour cream until smooth. Add the eggs, lemon zest, and vanilla, and process just until blended.
Add the softened butter and half the banana mixture to the flour, and mix on low speed until combined. Increase speed to medium and beat for 1-2 minutes; scrape down sides. Gradually add the remaining banana mixture in two batches, incorporating each addition for about 30 seconds before adding the next.
Pour the batter into the prepared pan, smoothing the top. Bake 30-40 minutes, until a toothpick comes out clean and the center springs back when pressed lightly.
Let the cake cool for about 10 minutes in the pan, then loosen the sides and finish cooling on a rack. Dust the top with powdered sugar.
Recipe by Sprung At Last at https://sprungatlast.com/2015/02/cordon-rose-banana-cake/