These cupcakes are best made the day before. To make lime curd; strain juice into a large heatproof bowl, add sugar and whisk to combine. Add egg yolks and whisk to combine. Put the bowl over a saucepan of gently simmering water and stir occasionally for 15-20 minutes until thick.
Remove bowl from saucepan and rest on bench. Add 5 cubes of butter at a time, stirring constantly until melted, smooth and glossy. Allow to cool for 15 minutes. Cover bowl with plastic wrap and refrigerate until chilled and thickened.
Preheat oven to 180°C. Line 18 holes of two standard-size muffin tins with paper cases. Sift flour and custard powder into a large bowl, add ¾ cup of the sugar and stir to combine. Whisk together buttermilk or milk, eggs, extract and melted butter.
Make a well in the centre of dry ingredients, add buttermilk mixture and whisk until smooth. Spoon into paper cases until three-quarters full. Bake for 25-30 minutes or until a skewer comes out clean when inserted into cakes. Leave in tins for 10 minutes before turning out onto a wire rack to cool.
To make meringue; put reserved eggwhites and remaining sugar in a medium glass bowl over a saucepan of simmering water. Stir until sugar dissolves, about 3-4 minutes. Transfer mixture to the small bowl of an electric mixer and beat for 6-7 minutes or until mixture forms firm glossy peaks.
Use the tip of a sharp knife to cut a V-shaped piece out of the top of each cake, about 2cm deep and 5cm wide. Spread 2 teaspoons of curd in each hole.
Preheat grill to high. Fill a piping bag fitted with a ½cm star pipe with meringue. Pipe meringue on top of each cake. Put cakes on an oven tray and place under hot grill for a few minutes until meringue begins to colour. Alternatively, use a domestic blowtorch.