Preheat the oven to 350°. Lightly spray an 8-by-12-inch baking dish with cooking spray. In a large bowl, beat the butter until creamy. Beat in the brown sugar and buttermilk until combined. Beat in the eggs and egg whites.
In a large measuring cup, mash the overripe bananas with a fork; you should have 1 cup of puree. Halve the ripe banana lengthwise; then cut it crosswise into 1/2-inch chunks and add it to the puree. Stir in the lemon zest and juice. Fold the banana mixture into the batter, then fold in the flour and baking soda just until no white streaks remain. Spoon the batter into the prepared baking dish and sprinkle the top with the granulated sugar.
Bake the banana cake for about 30 minutes until golden and the top springs back when lightly pressed in the center. Let cool completely in the pan before cutting into pieces.
One Serving Calories 276 kcal, Protein 5 gm, Carbohydrate 49 gm, Cholesterol 62 mg, Total Fat 6.9 gm, Saturated Fat 3.8 gm.