In a medium-sized bowl, combine the butter and 1/2 cup sugar. Add the eggs and continue mixing. In a separate bowl, mix together the flour, baking powder, salt and almond extract. Add the flour mixture to the butter mixture and mix well.
Spray a 9x13-inch baking dish with cooking spray and pour the batter into it. Spread it out evenly.
Slice the plums, removing the pit as you go. Layer the plum wedges on top of the batter in whatever pattern you choose.
Mix together 1/2 cup sugar and the cinnamon and sprinkle it over the plums. Bake in a preheated 375-degree oven for 45 minutes. Serve with whipped cream.