Fill a small saucepan with about 1 inch of water. Place over medium-low heat.

In a bowl that is large enough to fit over the pan without touching the water, off the heat combine the sugar and the lemon zest. Use your fingers to rub the zest into the sugar until it is light and fluffy. Whisk in the eggs, then the lemon juice.

Set the bowl over the simmering water and start whisking. Cook until the mixture reaches 180ºF using an instant read thermometer. Whisk constantly so that the eggs don’t scramble. The mixture will start out thin, but will start to get thicker as the mixture heats up. You will know that the mixture is up to heat when it has thickened and the whisk leaves tracks in the cream. This can take 10 minutes or longer.

As soon as the cream comes up to temperature, remove the bowl from the heat and pour the mixture through a strainer into a blender. Let the cream sit for a few minutes, until the temperature cools down to about 140ºF.

Turn the blender on high, and as it is blending, start adding the butter, one tablespoon at a time, scraping down the sides of the blender as needed. Once all of the butter has been added, let the blender run for 3 more minutes.

Pour the lemon cream into a container and press a piece of plastic wrap over the surface of the cream. Refrigerate at least 4 hours, or overnight.

Pour the flour, powdered sugar, and salt in a food processor and pulse a few times to combine.

Add the butter to the food processor and pulse until the butter is cut into the mixture. There will be pieces about the size of oatmeal flakes.

Break the yolk apart, then pour it over the mixture in the food processor. Start pulsing in longer intervals, 5-10 seconds each, until the dough starts to clump together.

Turn the dough out onto a counter and knead a couple of times, just enough to incorporate any dry ingredients that were not mixed in.

Butter or spray an 11-inch tart pan with nonstick cooking spray. Press the dough lightly into the pan and up the sides. Try to get it in as even of a layer as possible without overworking the dough. Place the pan in the freezer for 30 minutes.

While the crust is in the freezer, preheat the oven to 350ºF.

After removing the tart shell from the freezer, place a sheet of parchment paper over the top and fill with pie beads or dried beans. Place the tart pan on a baking sheet and bake the crust for 25 minutes. Remove from the oven and remove the parchment and the beads and return the crust to the oven for another 5 minutes, or until golden brown.

Allow the crust to cool completely.

Place the egg whites in the bowl of a stand mixer or in a large bowl. Whisk until they are frothy. With the mixer on, start adding the sugar, one tablespoon at a time until all of the sugar has been incorporated. Continue to beat on high until the mixture has stiff, marshmallow-like peaks. Add the vanilla and beat until mixed in.

Preheat the oven to 350ºF (if you do not still have it on from making the crust).

Remove the filling from the refrigerator and whisk a few times to loosen it up. Spoon the cream into the tart shell and smooth into an even layer.

Add the meringue to the top, making sure to spread all of the way to the crust.

Bake the tart until the meringue has slightly browned, about 10 minutes.