Preheat the oven to 190°C (375°F) and line a muffin tin with muffin cases.
Using a hand-held electric whisk or a freestanding electric mixer with the paddle attachment, beat together the butter, sugar, flour, baking powder, salt and lemon zest on a low speed to combine. Increase the speed to medium and keep beating until smooth and thick, scraping down the sides of the bowl from time to time. Pour in the remaining milk and eggs and continue to mix on a medium speed until all the ingredients are incorporated and the batter is smooth.
Spoon the batter into the paper cases, filling each by about two-thirds. If any batter is left over, use it fill up to four more cases in a second tin. Leave to cool slightly before removing from the tin and placing on a wire rack. The cakes will need to cool completely before you frost them.
To make the frosting, put the sugar into a small saucepan, cover with water (about 150ml) and bring to the boil. Meanwhile, using the electric whisk or freestanding mixer with the paddle attachments, whisk the egg whites until just foamy.
Allow the sugar to boil for just 5-10 minutes or until it has reached the soft-ball stage, then increase the mixer speed to medium and pour the sugar onto the egg whites. 9Be very careful as the sugar is extremely hot and will burn if it comes in contact with your skin.) When all the sugar has been added, increase the speed to high and whisk until the underside of the bowl feels lukewarm. The meringue should have quadrupled in size and be very white, smooth and fairly shiny.
Once the cupcakes have fully cooled, lay them out on a tray and hollow out the middle of each cake using a sharp knife. Make the hole about 2cm deep and 2cm wide and keep each cut out piece of sponge. Using a teaspoon, fill the holes about half full with the lemon curd and place the cut-out piece of sponge back on top, trimming to fit if needed.