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.
- Make one portion of the shortbread crust.
- 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.
- 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.
- 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.
- Let cool completely, remove from pan, then cut into rectangular pieces.