Preheat the oven to 350F and prepare a 10 inch cast iron pan by spraying with cooking spray or greasing with butter.
In a large bowl, beat the butter and sugars until combined and fluffy. You can use a stand or hand mixer, but you can also use a whisk and wooden spoon.
Once combined, add the egg and whisk to combine. The batter should be smooth.
Whisk in the vanilla, lemon juice, and lemon zest.
In a separate bowl, combine the dry ingredients (flour, baking soda and salt). Add half of the dry ingredients to your batter. Stir until combined. Continue to mix until fully incorporated.
Add all of the buttermilk. Stir to combine. The batter should be very smooth.
Add the remaining dry ingredients. Stir to form the final batter. It should be nice and smooth! It's a pretty thick batter.
Add the cake batter to the prepared pan and smooth the top with a rubber spatula. Arrange the plum wedges in circular patterns to cover the top of the cake. Press the plums lightly into the batter to hold their place.
Bake for 40-45 minutes, until the cake is golden brown on top and fully cooked through. If the top starts to get too dark, you can cover it with foil until the cake is fully cooked. The cake is cooked through when a cake tester inserted into the middle of the cake comes out clean.
Let the cake cool in the pan and then slice and serve!