Preheat your oven to 350°F. Line an 8×8-inch baking pan with parchment paper, leaving an overhang for easy removal, and lightly grease with cooking spray.
In a medium saucepan, melt the butter over medium heat. Stir in 3/4 cup brown sugar and cook, whisking constantly until. Let it bubble for about 30 seconds, then pour into the prepared pan.
Arrange the banana slices in a single layer over the caramel sauce, leaving a small space between each slice for even caramelization.
In a large mixing bowl, whisk together the egg, granulated sugar, oil, Greek yogurt, and vanilla extract. Mix in the mashed bananas.