Start by rolling out your pastry dough on a floured surface until it’s about 1/8-inch thick. Carefully transfer this to a 9-inch pie plate. Don’t worry if it doesn’t look perfect—just trim the crust to about half an inch beyond the rim of the plate and flute the edges to give it a beautiful finish. Once that’s done, pop it in the fridge for about 30 minutes while you preheat your oven to 425°F.
Next, line your chilled pie crust with a double layer of foil to keep its shape during baking. Fill it with pie weights, dried beans, or uncooked rice—this will help prevent the crust from puffing up. Bake it on the lower rack of your oven until the edges are golden brown, which should take around 20-25 minutes. Afterward, carefully remove the foil and weights and bake it for an additional 4 minutes or until the bottom is golden brown as well. Set aside to cool on a wire rack.
While your crust is cooling, let’s whip up that luscious grapefruit filling! In a large saucepan, combine sugar and cornstarch before stirring in your fresh pink grapefruit juice and water until everything is smooth. Cook over medium-high heat, stirring consistently until it thickens and bubbles up—this will take just a few minutes. Lower the heat and let it cook for another 2 minutes before removing from heat.
Take about half a cup of your hot filling and slowly stir it into the lightly beaten egg yolks to temper them. Once combined, return everything to the pan while stirring constantly, then bring it back to a gentle boil. Keep cooking and stirring for another couple of minutes until it's nice and thick again. Finally, remove from heat and gently stir in butter along with lemon extract.
In a large mixing bowl, beat your egg whites together with cream of tartar at medium speed until soft peaks begin to form. Gradually add sugar one tablespoon at a time while increasing your mixer speed to high—keep going until stiff glossy peaks form and all sugar is dissolved. This fluffy cloud will be what crowns our pie!
Pour the hot grapefruit filling into your cooled crust; be sure it's spread evenly. Then, take your whipped meringue and spread it over the top of the filling, sealing the edges well.
Bake your assembled pie in an oven preheated to 350°F until you see that gorgeous golden brown color on top of your meringue—this should take about 12-15 minutes. Allow it to cool on a wire rack for an hour before placing it in the refrigerator for at least three hours before serving.
