Prepare the topping

• Preheat the oven to 350 °F. Grease a 9-inch round cake pan with nonstick spray and line with parchment.
• Place the bananas cut side down on the parchment, arranging or trimming them to fit.
• In a saucepan, combine the granulated sugar and water over medium-high heat and cook, shaking the pan if needed, until an amber caramel forms.

Pour the caramel

• Immediately pour the caramel over the bananas in an even layer.

Make the batter

• In a large bowl, whisk the brown sugar and mashed bananas until a relatively smooth paste forms.
• Whisk in the olive oil, yogurt, vanilla and eggs until well incorporated.

Add the dry ingredients

• Add the flour, salt, baking soda, baking powder, cinnamon and nutmeg and stir until just incorporated with no pockets of flour.
• Fold in the dark chocolate chips and, if using, the walnuts.

Assemble and bake

• Pour the batter over the caramel-coated bananas and smooth with a rubber spatula.
• If using, swirl in the chocolate hazelnut spread.
• Bake until golden and cracked, with an internal temperature of 190 °F.

Unmold and serve

• Let cool for 15 minutes, then run a paring knife along the edges of the cake.
• Place a plate over the cake pan and invert, discarding the parchment paper.
• Let cool slightly, then serve warm.