Make the Plum Preserves. In a tall-sided pot (because the mixture will boil up), combine the plums, sugar, and lemon juice.
Heat over medium heat, stirring often, until the plums are very soft.
Skim any foam off the top, as necessary. Continue to cook until the mixture thickens, and most of the liquid has evaporated. The cooking process could take 25 to 40 minutes (or more) depending on how many plums and amount of sugar you used.
Scrape the preserves into a bowl to cool to just barely warm (or cooler if you have the time). You can place them in the refrigerator to speed up the process.
Make the Cake. Preheat the oven to 350 degrees. Grease a 10-cup Bundt pan and set aside.
In a stand mixer bowl, add the cake mix and pudding mix. Using the paddle attachment, mix the dry ingredients together. This can alternatively be done using a hand-held mixer. Mix for about 1 minute on medium-low.
Add the egg whites, sour cream, oil, buttermilk, and vanilla. Mix on medium speed for 2 minutes, scraping the bowl and paddle as needed.
Pour about ⅓ of the batter into the pan. Dollop about ¾ cup of the plum preserves on top of the batter. Try to keep the preserves from touching the edges of the pan. The preserves could stick to the pan and affect the outcome of a perfectly inverted cake.
Pour about another ⅓ of the batter over the preserves. Dollop the remaining preserves on the batter.
Scrap the last of the batter evenly on top.
Bake in the oven for 40 to 50 minutes, or until a toothpick inserted comes out with a few moist crumbs. Begin checking for doneness at 35 minutes if your oven runs hot.
Remove the cake from the oven and cool at room temperature for 15 to 20 minutes. You should see the cake releasing from the sides.
Invert the cake onto a flat cake stand or cake plate.
At this point, the cake can be eaten warm with the Plum Whipped Cream or the cake can be completely cooled and glazed.
Make the Plum Whipped Cream. In a stand mixer fitted with the whisk attachment, place the whipping cream, sugar, and vanilla in the mixer bowl. Turn the mixer on low, gradually increasing to medium speed, so as to not splatter. Once the cream is somewhat thickened, add the plum preserves and whip on high until stiff peaks. Serve with Plum Bundt Cake.