Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

Cream together the softened butter and sugar until light and fluffy.

Mix in egg yolk and vanilla extract until well combined.

Gradually add flour and salt, mixing just until a dough forms.

Scoop out tablespoon-sized portions of dough and roll them into balls, placing them on the prepared baking sheet.

Using your thumb or the back of a spoon, make a well in the center of each cookie.

Fill each well with lemon curd, being careful not to overflow.

Bake in the preheated oven for about 10 to 12 minutes or until the edges are set and just lightly golden.

Allow the cookies to cool on the baking sheet before transferring them to a wire rack.