Preheat oven to 350°F Butter or coat with cooking spray a 13x9 inch baking dish.
For the crust: In a food processor, add all the ingredients and pulse a few times. Pat the crust into the bottom of the prepared dish. Bake for 10-12 minutes or until golden brown. Let cool.
Cut the plums in half and remove the pit, cut the bananas in small slices, sprinkle with a Tb of sugar, set aside.
For the Cake: Mix the flour, cornstarch, baking powder and salt together and set aside.
In the bowl of stand mixer (or hand held mixer), cream the butter and sugar. Add the eggs, one at a time and beating well, scraping the bowl if necessary, after each addition.
Add the flour mixture, alternating with the buttermilk. Spread the batter over the shortbread base. Place the plums and bananas (if using) on top of the cake in rows of 4 or so, depending on their size. Do not worry if they seem to stick out, the batter rises around them. Bake for 50-55 minutes. Ready!