Preheat the oven to 190°C/170°C fan/375°F/gas 5. Line a 12-hole bun tin with baking cases. In a mixing bowl, beat the margarine with the sugar until pale and fluffy. Gradually beat in the eggs, then sift the flour and baking powder together into the bowl. Using a large metal spoon, carefully fold the dry ingredients into the egg mixture. Spoon a teaspoon of the mixture into the cake cases.
If you wish to include the raspberry jam, make a slight indent in the mixture with a dampened teaspoon and spoon a little jam evenly on top and then cover with the remaining cake mixture. Bake on the middle shelf in the oven for 15-18 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. Sift and beat in the icing sugar gradually to make a smooth, creamy icing. Transfer to a piping bag fitted with a 1cm star nozzle and pipe a generous swirl on top of each cake. Decorate with sugar butterflies and serve.