Preheat oven to 400°.
Make flax egg then set aside.
Combine flour, sugar, baking powder, and salt in a bowl; then, cut butter into the bowl, then add the flax egg and use your hands to combine everything together. The mixture shouldn't be too sticky--it should be slightly crumby but still able to hold together.
Grease an 8x8 baking dish with coconut oil, or use foil to line the dish, then press 3/4 of the mixture down into the bottom of the dish.
For the filling: add all ingredients to a bowl and stir to combine. Then, pour it over top of the crust. Sprinkle the remaining 1/4 crust mixture over top of the plum filling.
Bake bars for approximately 30 minutes, or until edges are slightly golden.
Remove from oven, then wait until they're cooled before cutting and serving. These bars stay the freshest if they're stored in the refrigerator. Feel free to sprinkle extra sugar over top if desired.