Preheat oven to 350°F. Lightly grease or spray a 9 or 10-inch cast iron skillet, 9x9 square baking dish or 9-inch round cake pan with vegetable cooking spray. Set aside.
To make the crust:
In a medium bowl whisk together 1 cup of sugar, baking powder, salt and flour. Add the butter and mix until lightly blended with a rubber spatula or wooden spoon. Add the egg and mix until the dough comes together and all ingredients are incorporated. Gently press two-thirds of the mixture into the prepared pan. Put the remaining dough in the refrigerator. Bake the crust for 5 minutes then remove from the oven to cool slightly.
To make the fruit mixture:
In a large mixing bowl, whisk together the sugar and cornstarch. Add the plums and blackberries and the lemon juice. Fold together using a rubber spatula to combine until all sugar is wet.
Pour the fruit over the lightly baked crust and spread evenly. Crumble the remaining chilled dough over the fruit.
Bake the bars at 350°F for about 40-45 minutes or until the top is just lightly browned and the fruit is bubbly. Remove from the oven and cool to room temperature or serve slightly warm. Cut into bars or wedges and dust with powdered sugar if desired, or serve with a scoop of ice cream.
Baking in cast iron may yield a darker colored crisper crust