Prepare the ingredients for the cake, leave butter soften in a room temperature.
Whisk butter together with sugar. Into the whisked mixture, add one egg at the time and continue beating until the mixture is fluffy.
In a separate bowl, combine together instant oatmeal, flour and baking powder. Add this mixture into the butter mixture and stir until it's well combined.
Spread the dough evenly over the bottom of the cake pan.
Place the half plum pieces at the edges of the cake.
Sprinkle oatmeal, coconut flakes and sugar mixture over the top of the cake.
Bake at 350F for 40 minutes or until it's golden brown.