How to Make It
To prepare the crust, preheat oven to 350°F. Pulse flour, powdered sugar, 2 tablespoons cornstarch, and 1/2 teaspoon salt in a food processor until combined. Top with butter pieces; pulse until mixture resembles coarse meal, 4 to 5 times. Drizzle evenly with oil; pulse just until moistened, about 2 times. (Mixture will be crumbly.) Transfer mixture to an 8-inch square glass baking dish coated with cooking spray. Pat into an even layer, being careful not to compact it. (Pressing too firmly will create a dense, tough crust.) Bake at 350°F until lightly browned all over, 25 to 30 minutes. Cool to room temperature.
To prepare topping, combine granulated sugar, grapefruit juice, 3 tablespoons cornstarch, and 1/4 teaspoon salt in a heavy saucepan. Bring to a boil over medium, stirring constantly. Cook, stirring often, until thickened, about 2 minutes.
Place egg yolks in a medium bowl. Gradually whisk in half of grapefruit mixture. Add egg mixture to remaining grapefruit mixture in pan. Bring to a boil over medium, whisking constantly. Remove from heat; whisk in Campari. Pour through a fine-mesh strainer into a bowl; discard solids.
Spread grapefruit mixture onto crust. Cover and chill until set, 3 to 4 hours. Cut into 16 squares.