How to Make It
Prepare an ice bath by filling a large bowl with ice; place a medium bowl on top of ice. Set aside. Whisk together egg yolks, eggs, granulated sugar, and bottled Key lime juice (such as Nellie & Joe's) in a medium saucepan over medium. Cook, whisking constantly, until sugar is dissolved, about 3 minutes. Whisk in cornstarch.
Increase heat to medium-high, and bring mixture to a boil, whisking constantly. Boil, whisking constantly, 1 minute. Remove from heat; whisk in unsalted butter, 1 tablespoon at a time, until butter is melted.
Pour mixture through a fine wire-mesh strainer into medium bowl over ice bath. Stir mixture often, using a rubber spatula, until cooled to room temperature, about 10 minutes. Remove bowl from ice bath; place plastic wrap directly on curd (to prevent a film from forming).
Chill completely, about 1½ hours. To serve, spoon over ice cream, and sprinkle evenly with graham cracker crumbs and lime zest.