Preheat oven to 180 C/ 350F. Place paper cases in a 12 hole muffin tin.
Cream butter and sugar together in a large bowl until light and fluffy.
Add the eggs, one at a time, beating well between additions.
Add the flour and salt, and combine thoroughly.
Add 1-2 tbsp lime juice so that the batter forms a soft dropping consistency.
Spoon into the prepared baking cases and bake for 15-20 minutes, until well risen and golden. A skewer inserted in the centre will come out clean when the cupcakes are cooked.
Transfer to a wire rack to cool completely.
To prepare the frosting, cream together the butter and icing sugar until smooth and light. Add the lime juice and incorporate well.
Pipe onto the top of the cooled cupcakes using a large star nozzle.
Sprinkle with lime zest and serve!
Notes
Calories per cupcake: 450 kcal (it's all that butter in the frosting!)