In a large mixing bowl, combine the sugar and lemon zest. Add the melted butter and stir until well combined. Add the eggs, vanilla extract, and salt, followed by the lemon juice. Mix until everything is well incorporated.
Sift the cake flour and baking powder into the wet ingredients. Gently fold everything together with a spatula until a soft, smooth dough forms. The dough will be very soft and sticky at this stage. Cover the bowl with plastic wrap and refrigerate for about 1 hour, or until the dough is firm enough to shape.
Once chilled, scoop about 1 tablespoon of cookie dough and roll it into a ball. Roll each ball generously in powdered sugar until completely coated. Arrange the dough balls on a lined baking sheet, leaving some space between them.
Using the back of a small measuring spoon, gently press a deep indentation into the center of each cookie. Pipe a little lemon curd into each indentation. Repeat with the remaining dough.
Bake in a preheated oven at 165°C (330°F) for approximately 12 minutes. The cookies will still be soft and may look slightly underbaked in the center, but they will firm up as they cool. Avoid overbaking to keep their texture soft and tender.
Remove the cookies from the oven and let them rest briefly on the baking sheet before carefully transferring them to a wire rack to cool completely.
Notes
Use Fresh Lemon Zest: Rub the lemon zest into the sugar before adding the melted butter to help release its fragrant citrus oils. This gives the cookies a brighter, more refreshing lemon flavor.
Let the Melted Butter Cool: Allow the melted butter to cool slightly before mixing it with the sugar and eggs. If the butter is too hot, it may partially cook the eggs.
Don't Overmix the Dough: Once you add the cake flour and baking powder, gently fold everything together until just combined. Overmixing can develop too much gluten, making the cookies less tender.
Chill the Dough Before Shaping: The cookie dough will be very soft and sticky after mixing. Refrigerate it for about 1 hour, or until firm enough to roll into balls. This makes the dough easier to handle and helps prevent excessive spreading during baking.
Coat Generously with Powdered Sugar: Roll each cookie dough ball in plenty of powdered sugar until completely covered. This creates a beautiful white, slightly crackled coating as the cookies bake.
Make a Deep Indentation: Use the back of a small measuring spoon to gently press a deep indentation into the center of each cookie. Make sure it's wide enough to hold the lemon curd without letting it overflow.
Don't Overfill with Lemon Curd: Pipe just enough lemon curd to fill each indentation without overflowing. Adding too much filling may cause it to spill over the sides as the cookies bake.
Avoid Overbaking: Bake the cookies at 165°C (330°F) for approximately 12 minutes. They will still be soft and may look slightly underbaked when you remove them from the oven, but they will firm up as they cool. Let them rest briefly on the baking sheet before transferring them to a cooling rack.