Drain mandarin oranges. Whisk butter with eggs and sugar. Add sour cream and mandarins. Combine flour, cornstarch and baking powder. Mix with sour cream mixture.
Pour mixture into buttered and floured pan. Bake in preheated oven at 200°C (approximately 400°F) for about 40-50 minutes (inserted toothpick should be clean). Cool in the pan. Remove cake from the pan and transfer to cake plate. Dust with powdered sugar. Beat cream until stiff and fill into a piping bag. Decorate cake with cream and orange wedges. Serve.