In a mixing bowl, whisk egg yolks with sugar until smooth and slightly thickened, and the mixture turns pale.
Pour lemon juice and lemon zest into a saucepan, then heat gently while stirring until just warm.
Gradually add the warm lemon mixture into the egg yolk mixture, whisking constantly to prevent curdling.
Return the combined mixture to the saucepan and cook over low heat, stirring continuously until it thickens slightly and coats the back of a spoon.
Remove from heat and stir in melted butter and a pinch of salt. Let it cool to room temperature.
Meanwhile, beat egg whites with vanilla extract until soft peaks form.
Gradually add caster sugar to the egg whites, continuing to beat until stiff, glossy peaks are achieved.
Preheat your oven to 375°F (190°C). Spoon the lemon custard into serving glasses or a dish, filling halfway.
Top the custard layer with a generous layer of meringue, spreading it gently with a spatula to create a smooth surface.
Bake in the oven until the meringue is golden brown, about 10-12 minutes, monitoring for even coloration.
Remove from oven and let cool slightly before serving. The meringue should be crisp on the outside while the inside remains soft and airy.