Line a 9 or 8 inch springform with parchment paper. Butter the sides and bottom.
Sprinkle the dark brown sugar evenly on the bottom of the springform and arrange sliced plums. *Note: You can also mix the sliced plums with the brown sugar and then arrange them onto the springform.
In a bowl combine flour, salt, baking soda, and cardamom(optional).
In a mixer with the paddle attachment mix the butter and granulated sugar for a couple of minutes. Add the sour cream and egg. Mix everything again. Add the flour mixture and mix just till the flour is incorporated, don't over mix. Transfer the batter on top of the plums and gently spread.
Bake for 20-25 minutes(dipanding on your oven) or till a toothpick comes out clean.
Let it cook for a few minutes before taking it out of the springform.
Add a plate on top of the cake and flip it upside-down. Remove the parchment paper. Enjoy!