Pre-heat the oven to 350°F.
Blend the first five ingredients in a medium size bowl with an electric mixer on medium speed until well combined. Add the flour and stir by hand until just combined.
Transfer the batter into a square baking pan that has been greased or sprayed with cooking spray.
Place the plums into the batter on their sides, sleeping close together in rows. Combine the cinnamon and sugar and sprinkle the mixture over the batter and plums.
Bake for 40 minutes. Do not overbake.
Serve warm with vanilla or butter pecan ice cream.