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.
- Preheat oven to 350F and line an 8” square pan with foil or butter a 9” round cake pan with removable bottom. (If using foil, it will help you lift the lemon bar out of the pan.)
- For the shortbread: Melt the butter and whisk in sugar and salt. Sift and mix in the flour.
- Press into a ¼” thick layer at the bottom of the prepared pan (you can make it curl slightly up the sides to help keep the lemon portion from sticking to the sides) and bake for about 25 minutes, until golden brown (this is a little crispier than normal shortbread cookies). Take out and set on a cooling rack. Prick the surface lightly all over with a fork - only halfway through the shortbread, not all the way through. This will help the filling to “stick”.
- For the filling: 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 pudding 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.