Preheat oven to 350 degrees.
Chop dried plums. Place in a small bowl and add 1 cup boiling water. Allow to stand 30 minutes.
Coat a 9-inch round pan with cooking oil spray and line with a large sheet of parchment paper so that it hangs over the sides of pan for easy removal of cake. Place in a large roasting pan.
Sift cocoa and combine with baking soda and salt with a whisk in a large bowl.
Puree dried plums with the soaking water in a blender until smooth. Add eggs, sugar and vanilla and puree until smooth. Drizzle melted chocolate through feed tube while running until fully combined.
Pour mixture into bowl with cocoa powder and combine well.
Spread batter into the prepared pan. Carefully pour boiling water into roasting pan to about an inch around cake pan.
Place in oven and bake until puffed and starting to crack, about 38-40 minutes. It may still look “jiggly.” Remove from water bath and cool. Place in refrigerator until firm, about 2 hours.