Heat oven to 180C/160C fan/gas 4. Beat half the sugar and all the butter until pale, then add whole eggs, one by one, beating in well each time. Stir in the zest and a squeeze of lemon juice, then fold in flour. Check the batter’s consistency; it should softly drop off the end of a spoon. Add more lemon juice if it’s a bit stiff.
Line a 12-hole bun tin with cake cases and put 1 large tbsp mix in each. Smooth then make a little dip in the centre. Spoon 1 tsp lemon curd into each dip and cover with 1 tbsp cake mix. Bake for 10 mins.
Meanwhile, place the egg whites into a clean bowl, whisk until stiff peaks form, then whisk in the rest of the sugar a tbsp at a time. Keep whisking until the mixture looks glossy and has thickened a little, like shaving foam. Spoon the mixture into a freezer bag and snip off the end to leave a hole with a 1.5cm diameter.
Take the cakes out of the oven - tops should be very pale and just firm - then pipe meringue all over the cakes. Return to the oven for 10-15 mins until the cakes are cooked through and meringue lightly browned. Can be made 1 day ahead.