Preheat the oven to 350F. Prepare your cake pans. Brush the bottoms and sides of your cake pans with melted butter. Line the bottoms and sides with parchment paper.
In a medium bowl, finely mash the bananas and set aside.
In a large bowl, combine the flour, baking soda and salt and whisk to combine.
In the bowl of a stand mixer fitted with a paddle attachment, combine the butter and sugar and beat on medium speed until light and fluffy, about 5-7 minutes.
Add the eggs one at a time, scraping the bowl after each addition. Beat in the vanilla.
Add in the flour mixture in three batches, alternating with the sour cream, stopping to scrape the bowl after each addition.
Add the bananas and beat until just combined.
Divide the batter into the prepared baking pans. Bake for 40-50 minutes, or until a toothpick comes out with a few cooked crumbs. Cool in the pan for 10 minutes.
Run a palette or butter knife around the cakes in their pans before turning them out to cool completely on cooling racks.
Wrap the cooled cakes in plastic wrap and chill in the refrigerator a couple hours to firm up.