01 - Melt the butter in a saucepan over medium heat. Continue cooking, swirling the pan occasionally, until the butter foams and golden-brown bits form at the bottom, approximately 5 to 7 minutes. Pour the browned butter into a bowl and let it cool for 10 minutes.
02 - Preheat the oven to 350°F (175°C). Grease a 9-inch round cake pan or loaf pan and line the bottom with parchment paper.
03 - In a medium heavy-bottomed saucepan, heat the granulated sugar over medium heat, stirring constantly until fully melted and deep amber in color. Carefully add the cubed butter, whisking until melted (the mixture will bubble vigorously). Slowly drizzle in the heavy cream while whisking continuously. Remove from heat and stir in the flaky sea salt. Let the caramel cool to room temperature.
04 - In a medium bowl, whisk together the all-purpose flour, baking soda, and sea salt. Set aside.
05 - In a large bowl, combine the cooled browned butter, granulated sugar, and packed light brown sugar. Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract, mashed bananas, and sour cream until the mixture is smooth and well blended.
06 - Gently fold the dry ingredients into the wet mixture just until combined. Be careful not to overmix, as this can result in a dense, tough crumb.
07 - Spread half of the batter evenly into the prepared pan. Drizzle half of the cooled salted caramel over the batter and swirl gently with a knife. Repeat with the remaining batter and caramel, creating a second layer with swirls throughout.
08 - Bake for 45 to 55 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
09 - Serve warm or at room temperature. Drizzle with additional salted caramel and garnish with sliced bananas or toasted chopped pecans as desired.