Beat the butter and sugar until frothy. Sift and mix the flour and baking powder, mix the lime zest, salt and chopped mint into the flour. Mix the eggs, sparkling wine, milk and syrup and stir alternately with the flour mixture into the butter and sugar mixture.
Put the dough in the muffin tin and bake in the preheated oven at 180 ° C for about 20-25 minutes. Then let it cool down well.
For the frosting, stir the butter with the powdered sugar until creamy. Mix the elderflower syrup, lime juice and lime zest with the butter and sugar mixture. Beat the cream cheese until smooth and carefully stir into the buttercream (do not beat).
Sprinkle the frosting on the cooled cupcakes and decorate with a straw, a lime wedge, a mint leaf and green sugar crystals.