Instructions

• Preheat oven to 350°F. Spray a 9-inch springform cake pan with cooking spray and set aside (a regular 9-inch round pan is likely too shallow and is not recommended).
• Melt ½ cup (1 stick) unsalted butter in a microwave-safe bowl (about 1 minute on high; stir and repeat in short bursts if needed). Let butter cool slightly, then pour it into the prepared pan. Use a clean finger or a small spatula to spread some butter up the sides of the pan so the bottom and sides are well greased (be careful — butter may be hot).
• Evenly sprinkle ¾ cup packed light brown sugar over the melted butter in the pan to form the caramel base.
• Prepare the bananas: slice each banana in half lengthwise, then cut each half into long slices (if a banana is very thick, cut each half into 3 or 4 long vertical pieces). Arrange the banana pieces, cut side down, in the pan in a fanned/spoked pattern so the bottom is covered with minimal gaps. Use short pieces to fill any gaps and around the outer edge so the bottom is evenly covered.
• In a large bowl whisk together 1 cup all-purpose flour, ¾ cup granulated sugar, 2 teaspoons baking powder, and a pinch of salt (if using). If you are using powdered buttermilk instead of liquid, add 2 tablespoons powdered buttermilk to these dry ingredients now.
• In a separate bowl whisk the wet ingredients: 1 large egg, ½ cup buttermilk (or if using powdered buttermilk, use ½ cup water here), ⅓ cup sour cream, 3 tablespoons canola or vegetable oil, and 2 teaspoons vanilla extract until combined.
• Pour the wet mixture into the dry ingredients and mix gently with a spoon or spatula until just combined. Small lumps are fine; do not overmix.
• Gently spoon or pour the batter over the arranged bananas, spreading it toward the edges without disturbing the banana pattern. Fill the pan to about 3/4 full. If you have extra batter beyond that level, discard it rather than overfilling the pan.
• Place the springform pan on a rimmed baking sheet to catch any potential leaks. Bake at 350°F for 38–40 minutes, until the top is lightly golden and the center is set and springy. Insert a toothpick about 1 inch into the center — it should come out clean or with a few moist crumbs but no raw batter.
• Remove the pan from the oven and set it on a wire rack. Cool the cake in the pan for at least 1 hour before inverting.
• To invert: run a thin knife around the inside edge if needed, unlock and remove the springform ring, place a serving plate larger than the pan over the cake, invert the pan and plate together, then lift off the pan base to reveal the caramelized bananas on top. If the topping sticks, allow the cake to cool longer before trying again.
• Optionally drizzle with salted caramel sauce before slicing and serving.
• Store leftover cake in an airtight container at room temperature for up to 5 days.