For the lemon curd:

1. In the bowl of a double boiler (the bottom of the bowl should just be touching the simmering water), add the eggs, lemon juice, lemon zest, salt, and sugar, and whisk until combined. Continue whisking while the curd is cooking. 
2. Cook and whisk for approximately 8 to 10 minutes or until an instant-read thermometer reaches 170 degrees and the mixture has thickened. 
3. Once the mixture has thickened, remove the bowl from the double boiler and, using a mesh sieve, pour the curd through the strainer and into a glass jar. Let the curd cool for a few minutes and then place it in the refrigerator for at least an hour or two to cool completely. The mixture should thicken a bit as it cools.

For the cookie dough:

1. Preheat the oven to 350 degrees.
2. Line two baking sheets with parchment paper.
3. In a medium bowl, mix the flour, baking soda, and salt. In the bowl of a stand mixer with the paddle attachment, beat the butter and sugar on high speed until fully blended and creamy, stopping to scrape down the sides regularly with a rubber spatula. 
4. Add the egg, egg yolk, and vanilla and beat on medium-high until combined. Pause to scrape down the sides with a rubber spatula as needed.
5. With the mixer set on low speed, slowly add the dry ingredients until combined, and the dough has formed. 
6. Refrigerate the dough for at least 30 minutes before making the balls. 
7. Make small balls of dough (roughly a generous 1 tablespoon). Roll the ball in the sugar and place the ball on a parchment paper-lined baking sheet, leaving plenty of room between the cookies. 
8. Using your thumb, press down in the middle of each cookie dough ball to form a cup or indentation. Place the cookie sheets with the cookie dough balls back in the refrigerator for 30 minutes. This will help the cookies keep their shape while baking. 
9. Place the chilled cookie sheets in the oven and bake for 15 minutes. Check the cookies at 10 minutes to make sure the bottoms of the cookies are not getting too brown.
10. When done, remove the baking sheets from the oven. If the bottom of the thumbprints has risen a bit, gently push down with your thumb again and carefully transfer the cookies to a wire rack to cool.
11. Once the cookies have cooled, using a small spoon, fill the indentations with lemon curd.
12. Using a small piece of cardboard with a straight edge, hold the cardboard over half the cookie and sprinkle with half with powdered sugar using a small mesh sieve.