Add the lemon zest, sugar, and salt to a double boiler and rub the zest into the sugar. Turn the heat on medium-high.
Add in the lemon juice and stir well. Continue to cook until the lemon juice starts to steam
Add the eggs and yolks to a separate small bowl and whisk.
Drizzle a third of the heated lemon juice into the eggs while constantly whisking the eggs.
Add the egg mixture into the lemon mixture and then continue to cook, stirring frequently, ensuring to scrap the bottom and sides of the bowl.
Cook until the mixture is visibly thickened and can coat the back of a spoon - about 8 - 10 minutes.
Strain the curd through a fine mesh strainer into an airtight container. Add in the butter and vanilla while the curd is still hot and mix well. Leave uncovered to cool to room temperature before placing in the refrigerator until needed.
Cookies
Beat the butter, cream cheese, sugar, and vanilla into a large bowl of a stand mixer fitted with a paddle attachment until light and fluffy - about 3 - 4 minutes.
Add in the eggs and beat well for an additional minute.
Mix together the flour, cornstarch, baking powder, and salt together in a small bowl.
Add the flour mixture into the dry ingredients and gently mix just until the dough comes together, scraping the sides and bottom of a bowl.
Cover the dough with plastic wrap and chill for at least an hour, or overnight. Preheat the oven to 350F and line two baking sheets with parchment paper.
Portion out the dough into balls that are about 2 ½ - 3 tablespoons big. Roll the dough balls in the powdered sugar
Create an indentation into the centers of the cookie dough and then fill with a teaspoon of lemon curd.
Bake for 10 - 12 minutes, or until the edges are a light golden brown.
Remove from the oven and let the cookies cool on the baking sheet for 2 - 3 minutes before transferring to a wire rack to cool completely
Notes
You can substitute the homemade lemon curd for store-bought. You may need to stir in a tablespoon of cornstarch or flour into the curd before portioning out and baking to ensure it is thick enough to withstand baking in the cookies.