2 sticks plus 5 tablespoons (21 tablespoons) butter, at room temperature and cut into tablespoon-sized pieces
1 recipe Sweet Tart Dough
Fresh sweetened and whipped cream for garnish, optional but recommended.
SWEET TART DOUGH:
1 1/2 cups all-purpose flour
1/2 cup powdered sugar
1/4 teaspoon salt
1 stick plus 1 tablespoon (9 tablespoons) very cold butter, cut into small pieces
1 large egg yolk
Have a thermometer, preferably an instant-read, a strainer and a blender set aside and ready to use. Bring a few inches of water to a simmer in a saucepan.
Put the sugar and zest in a large metal bowl that can be fitted into the pan of simmering water.
Off the heat, work the sugar and zest together between your fingers until the sugar is moist, grainy and very aromatic. Whisk in the eggs followed by the lemon juice.
Fit the bowl into the pan (make certain the water doesn’t touch the bottom of the bowl) and cook, stirring with the whisk as soon as the mixture feels tepid to the touch. You want to cook the cream until it reaches 180°F. As you whisk the cream over heat (whisk constantly to keep the eggs from scrambling) the cream will start out light and foamy, then the bubbles will get bigger, and then as the cream is getting closer to 180°F, it will start to thicken and the whisk will leave tracks. (*the tracks mean the cream is almost ready.) Don’t stop whisking and don’t stop checking the temperature. Be patient! Depending on how much heat you’re giving the cream, getting to 180°F can take as long as 10 minutes.
As soon as you reach 180°F, pull the cream from the heat and strain it into the container of a blender, discard the zest (*SEE NOTES). Let the cream rest/cool in the blender for 10 minutes.
Turn the blender to high and, with the machine going, add about 5 pieces of butter at a time. Scrape down the sides of the container as needed while you’re incorporating the butter. Once the butter is in, keep the machine going. Continue to mix the cream for another 3 minutes. If your machine gets a bit too hot, work in 1-minute intervals, giving the machine a little rest between beats.
Pour the cream into a container, press a piece of plastic wrap against the surface to create an airtight seal and chill the cream for at least 4 hours or overnight. When you are ready to assemble the tart, just whisk the cream to loosen it and spoon it into the tart shell.
Sweet Tart Dough:
Put the flour, powdered sugar and salt in a food processor and pulse a couple of times to combine. Scatter the pieces of butter over the dry ingredients and pulse until the butter is coarsely cut in. (You should have some pieces the size of oatmeal flakes and some the size of peas.)
Stir the yolk, just to break it up, and add it a little at a time, pulsing after each addition. Once the entire egg yolk is in, process in several 10-second pulses until the dough clumps together. Turn the dough out onto a clean counter and knead the dough lightly until the dry ingredients are just incorporated.
Butter a fluted tart pan with a removable bottom, either a 9- or 10-inch. Gently press the dough evenly over the bottom and up the sides. Freeze the crust for at least 30 minutes before baking (longer is even better).
Center a rack in the oven and preheat the oven to 375 degrees F.
Butter the shiny side of a piece of aluminum foil and fit the foil, buttered side down, tightly against the crust. (Since you froze the crust, you can bake it without weights.) Put the tart pan on a baking sheet to bake the crust for 25 minutes. Carefully remove the foil. If the crust has puffed, press it down gently with the back of a spoon. Bake for another 8 to 10 minutes or or until it is firm and golden brown. (Keep a close eye on the crust, it can go from golden to way too dark quickly). Transfer the tart pan to a rack and cool the crust to room temperature before filling.
If your lemon zest was very finely grated (I use a Microplane grater/zester), then you don’t need to strain the cream — just quickly scrape the hot cream from the bowl into the blender.
The recipe called for unsalted butter, but I used salted.