Heat the oven to 180°C (160° fan) 350°F gas 4.
Place 24 paper cases in mini muffin tins.
Beat the egg with the stem ginger and lime zest.
Beat in the sugar, oil, buttermilk and 2 tablespoons lime juice.
Mix the flour with the baking powder and bicarbonate of soda in a mixing bowl.
Beat the egg mixture into the flour until blended.
Add more buttermilk if necessary, to give a soft dropping consistency.
Spoon the mixture into the paper cases.
Bake for about 20 minutes until golden brown (test with a skewer or wooden toothpick; if it comes out clean, the cakes are done). Cool on a wire rack.
Whisk the cream and icing sugar until thick.
Spoon the cream into a piping bag and pipe swirls on top of the cupcakes.
Decorate with crystallised violets.