Line a 7" square (or similarly sized) baking pan with parchment paper (if not using a silicone pan).
Mix egg whites with dry ingredients (protein powders, cornmeal, almond meal, baking powder, and salt). Mix well.
Fold in 1 cup of plums.
Pour batter into pan.
Top with remaining plums.
Bake at 325F (163C) for about 30 minutes, until firm to the touch and a toothpick comes out clean.
Let cool slightly, and cut into 6 pieces.
Enjoy! Store in the fridge.
Notes
A note about substitutions: because each type of protein powder has a distinct flavor and texture when baked, I would only make the following substitutions. The rice protein can be replaced by pea protein or a pea blend (like Warrior Blend). Also, the whey+rice can be replaced by the same total volume in pea protein or a pea blend. (See my Recipe Basics video for examples of what goes wrong when you use the wrong protein powder.) Almond meal can be made by processing almonds in a food processor. And you can use regular plums in place of the sugar plums.