How to Make It
Prepare the Key Lime Curd: Whisk together lime juice, sugar, cornstarch, egg yolks, and eggs in a large saucepan. Bring to a boil over medium-high, and cook, whisking often, 1 minute. Add butter, 1 tablespoon at a time, whisking until melted and fully incorporated after each addition.
Pour lime juice mixture through a fine wire-mesh strainer into a bowl. Fill a large bowl with ice. Place bowl with strained lime juice mixture in ice. Using a rubber spatula, stir to cool.
Prepare the Cream Cheese Filling: Beat heavy cream in a bowl with an electric mixer on high speed until medium peaks form, 2 to 3 minutes. In another bowl, beat cream cheese, powdered sugar, and 2 teaspoons lime zest on high until light and fluffy, about 2 minutes. Add whipped cream to cream cheese mixture, and beat on medium until just combined, about 1 minute; set aside.
Assemble the cake: Place 1 layer of the shortbread cookies directly on a cake stand, arranging in a 8-inch circle. Top with a thin layer of Key Lime Curd. Sprinkle with a thin layer of pistachios; spread with a thin layer of Cream Cheese Filling. Repeat layers with remaining shortbread cookies, Key lime Curd, pistachios, and Cream Cheese Filling, ending with Cream Cheese Filling and reserving 2 tablespoons of the pistachios for top of cake. Chill until set, about 2 hours. Sprinkle top of cake with reserved chopped pistachios and desired amount of lime zest.