Negroni Dessert Bars

For the Shortbread Crust: Adjust oven rack to middle position and preheat oven to 350°F (175°C). Coat an 8- by 8-inch metal baking pan with cooking spray; line bottom and sides with parchment paper, leaving a 1-inch overhang.

In a food processor, pulse flour, powdered sugar, salt, orange zest, and butter until mixture forms a dry but pebbly meal, about 12 (one-second) pulses. Scatter into the prepared pan and use the palms of your hands to press firmly into an even, compact layer. Bake until lightly browned, 20 to 25 minutes.

For the Custard: While the crust bakes, in a medium saucepan, whisk together 3/4 cup sugar, orange zest and juice, Campari, lemon juice, and salt until well combined. Heat over medium, stirring occasionally, until sugar dissolves and mixture is steaming, about 4 minutes.

In a medium bowl, whisk together egg yolks, eggs, cornstarch, and remaining 1/4 cup sugar until well combined. Gradually pour hot orange juice mixture into egg mixture, whisking constantly, until combined; return mixture to saucepan. Cook over medium-low, whisking constantly, until mixture coats the back of a spoon (it should hold a trail when a finger is run through), and an instant-read thermometer registers 175°F (80°C), 4 to 6 minutes. Remove from heat.

Gradually whisk in butter, a few cubes at a time, until melted and well combined. Strain the mixture through a fine-mesh strainer set over a medium bowl, using a rubber spatula to press through as much custard as possible; discard solids.

Pour custard over hot crust and spread into an even layer using a small offset spatula. Bake until custard is firm, but with slight giggle in center, 14 to 16 minutes. Transfer baking pan to wire rack and let cool to room temperature, about 1 hour. Refrigerate, uncovered, until completely cool, at least 2 hours.

Using parchment overhang as handles, lift bars from pan and transfer to a cutting board. Cut into 16 (2-inch) squares, wiping the knife clean between cuts. Just before serving, dust with powdered sugar and garnish with orange slices, if using.