The Shortbread Crust:

1. Preheat your oven to 350 degrees.
2. In a food processor, process the flour, sugar and salt for about 10 seconds.
3. Then add the chunks of butter and process until the mixture looks like coarse crumbs.
4. Add the egg yolk and process again until the egg is thoroughly combined.
5. Press the mixture into a 10 inch tart pan (making the sides slightly higher than the centre because the lemon filling needs somewhere to sit).
6. Refrigerate for at least 25 minutes. Or you can put it into the freezer for 15 minutes.
7. Then bake for about 15-18 minutes, or until the crust is slightly browned. Make sure to prick the tart with a fork several times while baking, and watch it carefully.
8. Cool.
9. Remove from tart pan. Make sure to put your crust onto an oven-proof serving plate. Set aside.
10. Now on to the meringue.

The Meringue:

1. Now that your crust is done and set off to the side, preheat your oven to 400 degrees.
2. In a sauce pan set to med-high heat, whisk together the cornstarch and water. Whisk until the mixture is thick and clear.
3. Set aside. Cool.
4. In a mixer beat egg whites and cream of tartar until soft peaks form.
5. Then add sugar one tbsp at a time, while still beating.
6. Beat until the meringue looks glossy and has stiff peaks (see picture below). At this point add the cornstarch mix, and beat until thoroughly combined.
7. Set aside.
8. Now on to the filling.

The Filling:

1. In a saucepan over med-high heat, whisk together sugar, flour, cornstarch, salt, water, zest and lemon juice.
2. Mix until the liquid is boiling, then add butter and remove from the heat. Stir until combined.
3. In a separate small bowl, mix together 1/4 cup of the hot lemon filling with the 4 egg yolks. This is to prevent the eggs from cooking.
4. Then add this mixture to the remaining filling.
5. Bring to a boil, and stir for about 1 minute.

Putting it all together:

1. Pour the hot lemon filling into tart shell.
2. Spread on meringue, sealing the edges.
3. Bake in your preheated 400 degree oven until the meringue is golden brown (like a marshmallow). About 10 minutes. Watch closely.
4. Remove from oven, and let cool.