• Make the butter cookie tart shell first and store, wrapped, at room temperature until ready to fill with the lemon curd and decorate.

For the Lemon Curd

• In a medium-sized sauce pot, combine half the butter, half the sugar, the lemon juice, and zest. Bring to a boil over medium heat. Stir to dissolve the sugar.
• In a separate bowl, whisk together the egg yolks and remaining sugar.
• When the lemon juice mixture comes to a boil, remove from the heat. While whisking, gently pour about 1/3 of the lemon juice mixture into the yolks.

  PRO TIP: This process, called tempering, raises the temperature of the yolks without cooking them, preparing them to be better emulsified into the finished curd.
• Now, pour the tempered egg yolks back into the sauce pot with the rest of the lemon juice mixture. Return to medium heat and whisk constantly until the mixture thickens, about two or three minutes. It will boil in slow, big bubbles. You will be able to tell when it is done.

  PRO TIP: Be sure to scrape around the edges of the pot often while stirring to prevent "scrambled eggs" from forming on the rim. Don't worry if this happens, though. You will strain the curd.
• Remove the curd from the heat and fold in the rest of the butter.
• Strain the hot curd through a fine sieve into a clean, shallow bowl set over an ice bath or cool surface. You may need to use a spatula to press the curd through. Stir gently with a spatula until it cools to at least a warm room temperature. You should not be able to see steam coming off the curd.

To Assemble and Finish

• Pour the fresh curd immediately into the tart shell and gently smooth with an offset spatula, or cover and reserve for another use. Refrigerate.

  PRO TIP: If storing the lemon curd, make sure to place plastic wrap directly on the surface of the curd to prevent a "skin" from forming.
• Serve the tart plain, or decorate with powdered sugar or fresh fruit. Serve cool or at room temperature, refrigerating to store.