Preheat the oven to 190°C (170°C fan oven, 375°F, gas 5). Line 12 cupcake tins with white paper cake cases. In a mixing bowl, beat the margarine with the sugar until pale and creamy-light in texture.
Gradually whisk in the eggs, then sift the flour and baking powder. Using a large metal spoon, carefully fold the dry ingredients into the mixture.
Spoon a teaspoon of the mixture into the cake cases, and make a slight indent in the mixture. Spoon a little jam on top and then cover with the remaining cake mixture. Bake on the middle shelf in the oven for 16-18 minutes until risen, lightly golden and just firm to the touch. Transfer to a wire rack to cool.
For the icing, place the butter in a bowl and beat until soft. Gradually sift and beat in the icing sugar to make a smooth, creamy icing. Add rose water to taste.
Transfer to a piping bag fitted with a 1cm (1/2inch) star nozzle and pipe a generous peaked swirl on top of each cake. Decorate with sugar butterflies before serving.