Preheat oven to 350 degrees and line a half sheet baking pan with parchment paper.
Place butter, sugar, sour cream and bananas into stand mixer, mixing until well combined. Add flour, baking soda, cinnamon and salt, mixing until well combined. Transfer to prepared baking pan, spreading evenly.
Place melted butter into a medium mixing bowl, add brown sugar and cinnamon mixing well. Drizzle over cake batter then take a knife to swirl throughout. Bake for 35-40 minutes, or until baked through.
When cake is done baking, prepare brown butter icing. Brown butter in a small saucepan until bubbly and browned. Place softened cream cheese into a large mixing bowl. Pour browned butter over cream cheese whisking until smooth. Slowly add powdered sugar and almond milk until smooth. Drizzle over warm cake and spread evenly. Let cool completely then cut into squares and serve room temperature or chilled.