Lemon Bars

This started out as an experiment as I had read a Cook's Illustrated article on how to make an intensely lemon-y lemon bar without the fillers starting to get in the way. It's a tough balance between the liquid - the lemon juice - and the dry ingredients - the flour - because you can't really increase one without increasing the other. Adding more lemon means it becomes a runny mess unless you add more flour, but adding more flour simply cuts into the lemony taste.

Unfortunately, I've since lost the link to the Cook's Illustrated article, but it talked about a secret ingredient - cream of tartar. And from its detailed descriptions, I worked backwards from a standard lemon bar recipe and reconstructed the following for a crisp, crunchy shortbread topped with a truly lemony curd.

• one portion of shortbread crust
• ⅔ cup lemon juice (about 3-4 lemons)
• 3 eggs + 3 egg yolks
• 1 cup sugar
• ¼ tsp salt
• 2 tsp cream of tartar
• 2 tsp zest
• 2 tbsp flour
• 4 tbsp butter, cut into four 1 tbsp chunks

1. Make one portion of the shortbread crust.
2. While the shortbread is cooling, combine all filling ingredients except the butter in a double boiler over medium heat. Continuously whisk while the double boiler is simmering, until the mixture has thickened to about a custard consistency, with faint trails left behind by the whisk.
3. Remove the mixture from the heat and whisk in the butter one tbsp at a time, mixing until all the butter has been incorporated. Strain through a sieve into a clean bowl.
4. Pour over the shortbread crust in about a ½” thick layer and return to the oven for about 10 minutes. The filling should have only the slightest wobble to it when you shake it.
5. Let cool completely, remove from pan, then cut into rectangular pieces.