For the cookie dough:

1. Butter or use baking spray to grease cookie sheets or line them with parchment paper.
2. In a large bowl, cream (with a hand mixer or wooden spoon) butter and sugar until smooth. 
3. Add the egg and beat until well combined.
4. Add vanilla and mix.
5. Add sifted flour, cornstarch, baking powder and salt. Mix everything well. The dry ingredients can be sifted in another bowl before adding them if that's more convenient for you. I have them measured and sift them directly over the butter mixture. It will be a smooth and tender dough.
6. Take teaspoons of dough and form balls no more than 1 inch wide, the size of walnuts. Don't make large balls, as they will take too long to bake, and the filling will end up being too dry and probably spill.
7. Place them on the prepared baking sheets with space between them. 
8. Make an indentation in each piece of dough. The hole should not reach the bottom and break the dough. Tip: I use the bottom part of a small measuring spoon (1/2 teaspoon size). 
9. Put the whole baking sheet in the fridge for 30 minutes. This chills the dough so it doesn't spread or change its shape too much when it bakes.
10. Preheat the oven to 350°F/180°C.
11. Bake for about 7-8 minutes, until dry. Remove from the oven and fill with the lemon curd. If you feel the hole puffed up too much in the oven, lightly press it again with the back of the measuring spoon. Do this quickly, as the cookies need to bake some more. 
12. Bake for 4-5 more minutes, until they start to lightly brown and the curd is firmer. They might take more or less depending on your oven and the size of your cookies. 
13. Cool on a wire rack before eating. 

For the lemon curd:

1. Heat lemon juice and butter in a medium, heavy saucepan and remove just below the boiling point. 
2. Whisk eggs, yolks, and sugar to blend in a large bowl.
3. Gradually add the hot lemon mixture to the eggs, mixing well and quickly. Don't add it all at once as you can curdle parts of the eggs. 
4. Transfer the mixture back to the saucepan and stir constantly over medium-low heat until it thickens and leaves a path in the back of the spoon that stays put for a few seconds. Be careful not to burn the bottom.
5. Remove from the heat, add salt and vanilla, cover with a piece of plastic wrap or a freezer sheet that touches the whole surface, and let cool. This will prevent a thick layer from forming. 
6. Refrigerate in a jar or container with a lid for a week. Or freeze in an airtight container for up to a month.