For the shortbread crust:

1. Preheat oven to 350º.
2. Beat the butter, sugars, and vanilla on medium speed for about four minutes or until fluffy. You will need to stop the mixer and scrape down the sides a few times.
3. Then, with the mixer on low, slowly add the flour and beat until well blended.
4. Form the dough into a ball, cover it with a piece of plastic wrap, and chill for at least one hour.
5. On a floured surface, roll out the shortbread (refrigerated for at least 30 minutes) and press it into the tart pan. Make sure to press the dough up the sides of the pan. Using fork tines, gently poke holes over the bottom of the crust and bake for 20 minutes at 350º.
6. Remove from oven and place on a wire rack to cool.

For the cranberry curd:

1. While the crust bakes, in a medium saucepan large enough to hold the cranberries, add the sugar, salt, and lemon zest and stir. Add the cranberries and the 1 1/4 cups of water and bring to a boil over medium-high heat. Boil until all the cranberries have popped. This takes about 10 minutes.
2. Once the cranberry mixture is a thick syrup, drain it through a sieve. Using the back of a spoon, push the liquid through the fine mesh strainer and extract all of the juice. You’ll have exactly two cups. Discard the solids.
3. Add the cornstarch and cold water to the same pan and whisk until smooth. Add the egg yolks and whisk again until smooth, and then add the cranberry mixture.
4. Boil again over medium heat, stirring until thickened, about 5 minutes. Turn off the heat, add the lemon juice and cold butter, and mix until combined.
5. Pour the tart cranberry curd into the cooled tart crust in an even layer in the pan. Once cool, refrigerate for four hours or up to three days. On the day of serving, before topping with meringue, bring it back to room temperature.

For the Meringue:

1. In a stand mixer or hand-held electric mixer, beat the egg whites and salt on low speed until they are frothy/foamy.
2. Add the cream of tartar, adjust the speed to medium, and beat until the egg whites are somewhat fluffy with large bubbles around the outside.
3. With the mixer running, slowly pour in the sugar and continue to beat until all the sugar is combined.
4. Keep beating and as soon as peaks begin to form, add the vanilla.
5. Once the vanilla is incorporated and you have stiff peaks, stop beating.
6. Spread the meringue carefully over the top of the tart.
7. Using a kitchen torch, toast the meringue evenly until you’ve achieved the desired look.