Mash 3 bananas in a large bowl with a fork or potato masher until smooth (1 to 1 1/4 cups mashed banana). Whisk in the melted butter, 1 large egg, 1/2 cup packed light brown sugar, 1/2 cup Greek yogurt or sour cream, 1 teaspoon vanilla extract, and 1/2 teaspoon kosher salt.