Plum Upside-Down Cake

Okay...the batter in this cake is amazing. The plums were still on the sour side when I made this, but I actually found them to be a nice compliment to the sweeter cake. Saw this during a browse session on Real Simple. Next time...I'll try apricots.


Ingredients
1/2 cup butter, at room temperature + 1 tbsp. extra (reserved)
4 firm plums, eight pieces each
1/4 cup + 2/3 cup granulated sugar
1 cup all-purpose flour
3/4 tsp. baking powder
1/4 tsp. baking soda
1/4 tsp. salt
1 egg
2/3 cup sour cream
1 tsp. vanilla extract


Directions
1. Preheat the oven to 350 degrees. Line a cake pan with parchment paper (or grease well).
*Note: the recipe called for an 8" pan, but I only had a 9" one. It worked fine...my cake was just wider and shorter.
2. Now, prepare the plums. Slice them in half and remove the pit. Then slice each half into four equal sections. You should end up with eight slices from each plum.
*Note: I tried to slice them thinner, thinking I could make tighter concentric circles (as you'll see below), but I wouldn't recommend this. If the slices are too thin, they will break down when cooked in the skillet in the next step. Stick with the eight-slices-per-plum rule.

3. In a large skillet, melt the extra 1 tsp. butter. Over medium-high heat, add the plums and 1/4 cup of sugar. Cook for three to four minutes, stirring occasionally, until the sugar dissolves and the liquids have a syrupy consistency.

4. Arrange the plums in concentric circles in the bottom of the cake pan. It's easiest if you start with the outer circle first and work your way into the middle.
5. In a small mixing bowl, combine the flour, baking powder, baking soda and salt.
6. In a medium large mixing bowl, with a stand- or hand-mixer, beat together 1/2 cup butter and 2/3 cup of sugar until fluffy.
7. Add in the egg, sour cream and vanilla. Beat until combined.
8. Add the flour mixture, little by little, until just incorporated.
*Note: the batter is going to be very, very thick. So thick, in fact, I wondered if I had done something wrong. But rest assured - it's just really good!

9. "Pour" the batter over the plums...this is actually going to be more of a dump, to be honest. Smooth it out as gently and as best you can so that it covers all the plums.
10. Bake for 55 minutes, and let cool for one hour before inverting.
11. Invert cake onto a plate, slice, and serve.