Preheat the oven to 350 degrees F (180 degrees C). Line a 9x13 baking dish with parchment paper, and spray with cooking spray.
Cut each Plum (8) into 8 slices. Set aside.
In an electric mixer, mix the Egg (5) med-high speed, and gradually add the Granulated Sugar (1 cup) until pale and fluffy, at least 5 minutes.
Add the Sour Cream (1 cup) and Lemon (1/2 teaspoon) the mixer.
Sift and add the All-Purpose Flour (1 1/3 cup) and Baking Powder (1 teaspoon) into the batter.
Gently whisk them until mixed well.
Pour the batter into the prepared baking dish.
Top with the plum. Poke some of the plum into the batter and place more plums on top. Most of the plums should be covered with the batter.
Bake in the preheated oven for 40-50 minutes.
Invert the coffee cake into a cooling rack and peel off the parchment paper. Let cool for about 5-10 minutes.
Cut into small slices and serve.