In a medium bowl, cream the butter and slowly add the sugar. Beat until light and fluffy.
Add the eggs one at a time, mixing until incorporated.
Sift the flour and baking powder into the sugar mixture. Add salt and mix until just combined.
Add the vanilla, milk, and lemon juice, and mix until incorporated.
Scoop mixture into a lined cupcake tin about ¾ full.
Bake 12-15 minutes, or until a toothpick comes out cleanly from the center of the cupcakes. Let cupcakes cool.
Prepare the lemon curd by whisking the yolks and sugar in a heatproof bowl over a pot of simmering water.
Once the sugar is dissolved, add the lemon juice and zest. Stir until the mixture thickens, about 10 minutes.
Once the curd coats the back of a spoon, add the butter and stir until melted.
Remove from the heat, cover with plastic wrap, and refrigerate. Make sure the cling film touches the surface of the curd so that a film doesn’t form on top.
Prepare the frosting by whisking the egg whites and sugar in a heatproof bowl over simmering water until it thickens and the sugar dissolves completely. Tempering the eggs will ensure the frosting is food safe without being baked.
Remove from the heat, add the cream of tartar, and beat about 10 minutes until soft peaks form.
Using a piping tip or a knife, cut a hole into the top of the cupcakes and press down to make a well for the curd.
Add lemon curd into the cupcakes and frost them with the meringue.
Torch the frosting or carefully broil for about 3 to 4 minutes. Be sure that you use foil cupcake liners if broiling!