Line two loaf pans (10-inch by 3-inch or similar) with parchment paper, using enough to make 'handles' to lift cake out after baking. Spray them with cooking spray. Cover the bottom with an even layer of plum slices.
In a medium bowl, whisk together all dry ingredients. In a large bowl, whisk together all remaining ingredients. Add the dry ingredients to the wet ingredients and whisk until combined.
Divide the batter between the two pans and bake. Begin checking for doneness at about 30 minutes, and bake until the tops are golden brown and a toothpick stuck in the center comes out clean.
Let cool slightly in the pan and then use the parchment paper to carefully lift it out of the pan. Invert onto a cooling rack.