Cream together sugar, butter, eggs and salt until light and fluffy
Add in flour and baking powder.
Mix until combined.
Pour half of batter into 9 inch greased floured pan
Place half of plums on top of batter, cut side up
Spread remaining batter over plums and then top with remaining plums, cut side down.
Top with 1 Tbsp cinnamon sugar
Bake 350 for about 45-60 minutes or until a toothpick comes out clean
(baking varies depending on moisture in plums)