In a saucepan, add all the ingredients except the butter.

Cook over medium heat, whisking continuously, until the mixture thickens and coats the back of a spoon with a thick film.

Remove from heat and strain through a fine mesh strainer to remove the zest. Set aside to cool.

In a bowl, combine the flour, salt, and baking powder.

In the bowl of a stand mixer fitted with a paddle attachment (or use a hand mixer or a whisk), add the sugar, lemon zest, butter, and vanilla extract. Beat for 1–2 minutes or until fluffy. Add the egg and lemon juice, and beat until well incorporated.

Add the dry ingredients and mix on low just until no streaks of flour remain.

Wrap the dough in plastic wrap and freeze for 20–30 minutes, or until firm enough to shape into balls. Alternatively, refrigerate the dough overnight.

Preheat the oven to 180C (355F) and line a baking sheet with parchment paper.

Form 13 dough balls and roll them in icing sugar.

Place them on the baking sheet with at least 4 cm (2") of space between each one.

Using the back of a 1 teaspoon measuring spoon, make an indentation in the center of each dough ball. Fill each indentation with lemon curd.

Bake for about 13 minutes, or until the cookies have spread and the edges are golden and crinkly.

Remove from the oven and let cool for 5 minutes on the baking sheet before transferring to a cooling rack to cool completely.