Preheat oven to 350°. Lightly grease a 9-inch tart pan.
In the work bowl of a food processor, place vanilla wafers and almonds; pulse until almost finely ground. Transfer to a medium bowl; add melted butter, stirring to combine. Press mixture into prepared pan.
Bake until golden brown, 7 to 8 minutes. Let cool completely.
In a medium saucepan, heat grapefruit juice over medium heat until reduced to 1½ cups, about 30 minutes.
In a medium bowl, whisk together sugar, cornstarch, and zests; whisk in eggs. Gradually add hot grapefruit juice, whisking constantly. Return mixture to saucepan; add butter, whisking until butter is melted. Cook, whisking constantly, until thickened. Pour into prepared crust.
Cover with a piece of plastic wrap, pressing wrap directly onto surface of custard. Refrigerate for 3 to 4 hours. Garnish with whipped cream and Candied Ruby Red Grapefruit Rind, if desired.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/citrus-tart/