Prepare the Oven: Preheat the oven to 190℃ / Fan 170℃ / 375℉ / Gas 5. Line 9 wells of a muffin tray with cupcake cases.
Cream the Butter Sugar: Place the softened butter and golden caster sugar in a mixing bowl and beat until the mixture is very pale and fluffy.
Add the Lime: Beat the finely grated zest of 1 lime into the creamed mixture until well combined.
Incorporate the Eggs: Gradually add the lightly beaten eggs, a little at a time, beating thoroughly after each addition. Beat in the juice of ½ lime.
Fold in the Flour: Sift the self-raising flour and baking powder into the bowl. Gently fold them through the mixture using a spatula or large metal spoon until combined.
Fill the Cases: Divide the batter among the 9 cupcake cases, filling each one approximately half to two-thirds full.
Bake the Cupcakes: Place the tray in the centre of the preheated oven and bake for 20–23 minutes, until the cupcakes have risen and feel springy when gently touched.
Cool the Sponges: Transfer the baked cupcakes to a wire rack and allow them to cool.
Prepare the Sugar Syrup: Put the caster sugar and water into a heavy-based saucepan. Heat gently until the sugar has completely dissolved, then increase the heat and boil the syrup until it reaches 110℃ (230℉).
Begin Whipping the Egg Whites: While the syrup is heating, place the egg whites in a very clean bowl and whisk until they form stiff peaks.
Heat the Syrup Further: Continue heating the sugar syrup until it reaches 121℃ (249℉), then remove the pan from the heat.
Combine the Syrup Meringue: With the whisk running on its lowest setting, slowly pour the hot sugar syrup into the whipped egg whites. Avoid pouring the syrup directly onto the beaters, as it can splash back.
Whip the Meringue: Once all the syrup has been incorporated, increase the mixer speed and continue beating for at least 5 minutes, until the meringue is very thick, glossy, and cooled.
Create the Cupcake Centres: Using a small paring knife, carefully cut a plug from the top of each cooled cupcake to create a hollow.
Fill the Cupcakes: Spoon 1 teaspoon of lime curd into the hollow in each cupcake.
Pipe the Meringue: Fit a large star nozzle onto a piping bag, fill it with the Italian meringue, and pipe a generous swirl over each cupcake.
Toast the Meringue: Carefully brown the piped meringue using a blow torch. Alternatively, preheat the oven grill and place the cupcakes underneath for 1–2 minutes, positioning them in the centre so they are not too close to the grill element. Watch them closely, as the meringue colours very quickly, and rotate the cupcakes after about 1 minute if needed.
