Step 1In the bowl of a food processor, combine the butter and sugar. Pulse until the mixture resembles coarse, pea-sized crumbs.
Step 2Add the egg yolk, zest and juice to the bowl and pulse until combined. Add the flour and pulse until the mixture comes together to form a ball. Flatten the dough to one-fourth-inch thickness and wrap in plastic. Freeze for at least 1 or up to 24 hours.
Step 3Remove the dough from the freezer and heat the oven to 375 degrees. Allow the dough to sit at room temperature until it is pliable but still very cold. On a well-floured surface, roll the dough to a one-eighth-inch thickness. Lightly spray 6 (4-inch) tart pans with removable bottoms, and cut out 6 (5-inch) circles. Press the circles of dough into the pans and trim the dough to fit.
Step 4Line the tart shells with foil and fill with dried beans or pie weights. Bake the crusts until the edges are pale gold, about 15 minutes. Remove the foil and pie weights and return the tart crusts to the oven. Bake until the crusts are golden brown, about 5 to 10 minutes. Cool completely before filling.
Step 1Place the eggs, egg yolks, sugar, zests and juices in a bowl and whisk to combine the ingredients. Transfer the mixture to a double boiler and begin heating at a medium-low temperature. Do not let the mixture become too hot, or the eggs will start to scramble. Stirring frequently with a rubber spatula, cook the mixture until it thickens enough to mound on itself, about 20 to 25 minutes.
Step 2Place the butter in a bowl. Strain the egg-grapefruit mixture through a fine-mesh sieve into the same bowl. Stir to melt and incorporate the butter. Pour the filling into the cooled tart crusts. Refrigerate the tarts for at least 3 hours before serving.
Step 3At least 20 minutes before serving, bring the tarts out of the refrigerator. Garnish with whipped cream and candied grapefruit peel if desired.