Lemon curd:
In a saucepan, combine the lemon juice, zest, and half of the sugar and bring to a boil.

In a bowl, whisk together the eggs and remaining sugar.
Gradually add the hot liquid to the eggs, whisking constantly.
Return the mixture to the saucepan and cook over low heat, stirring constantly, until the mixture thickens.

Remove from heat, transfer to a container, and add the cubed butter.
Emulsify the lemon curd with an immersion blender.

Once the butter is fully incorporated and the curd is smooth, strain it through a fine-mesh sieve into a bowl, cover with plastic wrap, and refrigerate for at least 8 hours.

Mini cupcakes:
Preheat the oven to 325 F (160 C).
Line a mini muffin tin with paper liners.

Combine the flour, baking powder, and salt in a bowl.
In another bowl, cream together the butter and sugar, then add the eggs one at a time, whisking until smooth.
Add the flour mixture, lemon curd, and lemon zest, and mix until well combined.

Divide the batter among the muffin cups and bake for 15-20 minutes or until the cupcakes are firm to the touch.
Let cool for a few minutes, then transfer to a wire rack to cool completely.

Meringue:
Beat the egg whites with an electric mixer until stiff peaks form, gradually adding the sugar to stabilize them.
Beat until the meringue is stiff and shiny.

Assembly:
Spoon a generous amount of lemon curd onto each cooled cupcake, followed by a rosette of meringue using a piping bag fitted with a star tip.

Quickly torch the meringue rosettes with a kitchen torch to caramelize them.