Spray and line with parchment paper your baking pan. Make a dry caramel melting 5 tbsp of sugar in a non-stick saucepan. Pour it with care into the prepared baking pan, coating the bottom.Rinse and pat dry the plums. Cut them in halves and remove the pits. Place them over the caramel, with the cut down. Sprinkle with cinnamon to taste.Preheat the oven at 340 degrees F (170 degrees C).
Beat the egg whites with a pinch of salt until firm peaks are forming. Add 1/2 of the sugar and continue beating until becomes a meringue. Separately, beat the egg yolks, the rest of the sugar, the oil and vanilla extract until it becomes a smooth, silky cream.Incorporate gently with folding moves this yellow cream into the meringue, then the sifted flour and the baking powder. Smooth the surface and bake the cake for about 30 minutes at 340 degrees F (medium heat).
Let the cake cool for 10 minutes, run a knife to loosen the edges and flip it onto a serving platter. Remove the baking paper. Cut / slice it when completely cooled down. Decorate with icing sugar (optional).