A blend of ricotta and cream cheeses keeps these bars light and creamy. The filling is flavored with instant espresso powder, which can be found in the coffee section of well-stocked supermarkets. It’s more intensely flavored and dissolves better than instant coffee granules. To make ahead: Refrigerate finished cheesecake in pan wrapped in plastic wrap for up to one day. Cut into bars before guests arrive, and keep refrigerated until ready to serve.
1. To make Crust: Place 1 rack in center and 1 rack in bottom position of oven. Preheat oven to 350°F. Line 9-inch square baking pan with parchment paper, letting ends hang over sides of pan. Spray with cooking spray.
2. Pulse cookies in food processor until chopped into coarse crumbs. Add butter and sugar. Blend 1 to 2 minutes, or until crumbs are moist and stick together when pressed. Press mixture into bottom of prepared pan, but not up sides. Bake 12 minutes in center of oven, or until golden.
3. To make Filling: Whisk together egg, egg white, and espresso powder in bowl until espresso powder dissolves. Place ricotta, cream cheese, sugar, cornstarch, egg mixture, and salt in food processor, and blend until smooth. Spread Filling over hot Crust.
4. Slide baking sheet onto bottom shelf of oven to diffuse heat. Bake Filling and Crust in center of oven 30 to 33 minutes, or until Filling is just set in center and small cracks start to appear at edges. Transfer directly to refrigerator, and chill, uncovered, 6 hours, or until cold and set.
5. To make Frosting: Microwave chocolate and 3 Tbs. water in 2-cup glass measure 25 to 30 seconds, or until chocolate is soft. Add sugar, espresso powder, and salt; whisk until smooth. Spread warm Frosting over chilled cheesecake. Chill until Frosting is firm.
6. Lift cheesecake out of pan using parchment. Gently remove parchment from cheesecake. Transfer cheesecake to cutting board, and cut into 24 bars (3 rows of 8 bars). Transfer to serving plate.