Preheat the oven to 190ºC, 375ºF, gas 5. Line a 12-hole fairy cake tin, or a 24-hole tin for mini cakes, with 12 (or 24) paper cases.
Cream the butter in a large bowl or in an electric food mixer until soft. Add the sugar and vanilla extract and beat until the mixture is light and fluffy.
Add the eggs, one at a time, beating continuously, then sift in the flour and baking powder. (If you are pushed for time, and the butter is sufficiently soft, put the butter, sugar, eggs, vanilla, flour and baking powder into an electric food mixer and briefly whiz just until the mixture comes together.)
Divide the mixture evenly between the paper cases using two teaspoons for mini cakes or two dessertspoons for larger ones. Cook in the oven for 8-12 mins, or until golden and springy to the touch. (The mini cakes may take just 5 mins.) Transfer the cakes to a wire rack to cool.
When cooled, cut the top off each cake, then cut the tops in half to make the butterfly wings. Set aside. Spread half a teaspoon or so of jam on the top of the cut cake, then spoon or pipe the crème Chantilly or buttercream on top of the jam. Arrange two butterfly wings at an angle on top of each cake. Dust generously with icing sugar and decorate with dragèes, if using.