01 - In a saucepan, melt 1/2 cup unsalted butter over medium heat. Continue cooking, swirling occasionally, until it foams and golden-brown bits form at the bottom of the pan, about 5-7 minutes. Pour the browned butter into a heatproof bowl and let cool for 10 minutes.
02 - Preheat 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 1 cup granulated sugar over medium heat, stirring constantly until fully melted and deep amber in color. Carefully add 6 tablespoons cubed butter — it will bubble vigorously. Whisk until the butter melts completely. Slowly drizzle in 1/2 cup heavy cream while whisking continuously. Remove from heat and stir in 1 teaspoon flaky sea salt. Let the caramel cool to room temperature.
04 - In a medium bowl, whisk together 2 cups all-purpose flour, 1 teaspoon baking soda, and 1/2 teaspoon sea salt. Set aside.
05 - In a large bowl, combine the cooled browned butter, 1 cup granulated sugar, and 1/2 cup packed light brown sugar. Add 2 eggs one at a time, beating well after each addition. Mix in 1 teaspoon vanilla extract, 1 cup mashed ripe bananas, and 1/2 cup sour cream until the mixture is smooth and well blended.
06 - Add the dry ingredient mixture to the wet ingredients, folding gently with a spatula just until combined. Do not overmix — some small lumps are fine.
07 - Spread half of the batter evenly into the prepared pan. Drizzle with half of the cooled salted caramel sauce and swirl gently with a knife. Repeat with the remaining batter and caramel, creating a second layer with swirls.
08 - Bake at 350°F for 45-55 minutes, or until a toothpick inserted into the center comes out clean. Let the cake 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 banana or toasted chopped pecans as desired.