Mix the butter and sugar, then add the eggs one at a time. Sift the flour with baking powder and add butter to the 4/5 cup of sugar and eggs. Grate the orange zest and add it to the mixture.
Grease a not-too-large oven dish and bake in a preheated oven at 356°F (180 °C). Squeeze the orange for its juice and boil it with the 1/3 cup sugar until thickened.
Leave it to cool and pour the resulting sauce over the finished cake. Sprinkle it with grated chocolate or colored sprinkles. Enjoy!