Allow the butter, egg, and sour cream to come up to room temperature. This will take approximately 20 to 30 minutes.

Wash and slice the plums thinly (a little greater than 1/4 inch thick). Set them aside for now.

Preheat the oven to 350°F. Grease a 9 inch pie plate with baking spray or shortening.

Combine the flour, baking powder, and salt in a small bowl and whisk well.

Cream the butter and sugar on medium speed until light and fluffy. This will take anywhere from 4 to 8 minutes depending on the speed and strength of your mixer. This is most easily done with a stand mixer or a handmixer.

Add the egg and continue to beat on low medium speed until completely incorporated.

Add the sour cream and vanilla extract, then mix again until completely incorporated.

Add about half of the dry ingredients to the wet, stir a bit to incorporate, then add the rest and stir until everything is just incorporated. No dry streaks should remain. Do not over mix!

Transfer the thick batter to the prepared pie plate. Spread it evenly into a level layer.

Place the sliced plums on top of the batter as shown, overlapping them slightly as you form a circle around the edge of the batter.

Complete the second circle of plums, again overlapping the slices. Add an inner circle as well.

For the topping, combine the sugar and cinnamon in a small bowl. Sprinkle the topping over the plums.

Bake at 375°F for 45 minutes, or until an inserted toothpick tests clean.

Slice and serve as is, or with a bit of whipped cream or ice cream on the side. Refrigerate any leftovers.