Lightly coat an 8x8-inch-square baking pan (at least 2 inches high) with cooking spray. Line the bottom and two opposite sides of the pan with a sheet of parchment paper long enough to extend over the sides by a few inches to form a sling.

Process 9 ounces Oreo sandwich cookies in a food processor until fine crumbs form, 45 to 60 seconds. Drizzle in 5 tablespoons melted unsalted butter and pulse until evenly moistened, about 6 (1-second) pulses. (Alternatively, place the cookies in a resealable plastic bag and crush with a rolling pin into fine crumbs. Transfer to a medium bowl and stir in the butter.)

Transfer to the baking pan and use the bottom of a flat measuring cup or your hands to firmly press it into an even layer. Freeze for at least 10 minutes. Meanwhile, make the ganache.

Place 6 ounces coarsely chopped semi-sweet chocolate in a medium bowl. Place ¼ cup heavy cream in a small microwave-safe bowl and microwave until hot but not bubbling, about 40 seconds.

Drizzle the cream over the chocolate. Let sit for 3 minutes. Stir until the chocolate is completely melted and the mixture is smooth. Let cool until just warm to touch, about 5 minutes.

Pour the ganache over the crust and spread into an even layer, completely covering the crust. Refrigerate while you prepare the filling.

Whisk 8 ounces very soft room temperature cream cheese, ½ cup of the powdered sugar, 2 tablespoons Baileys Original Irish Cream, 1 tablespoon cocoa powder, and 1 ¼ teaspoons instant espresso powder together in a large bowl until smooth.

Beat 1 ½ cups cold heavy cream and the remaining ⅓ cup powdered sugar together in a medium bowl with an electric hand mixer on medium speed until medium peaks form, 3 to 4 minutes. (Alternatively, whisk together by hand.)

Transfer 2 cups of the whipped cream to the cream cheese mixture and gently fold together with a flexible spatula until combined and no streaks remain. Spread the mixture into an even layer over the ganache.

Cover and refrigerate until set, at least 4 hours or up to overnight. Cover and refrigerate the remaining whipped cream.

Run a thin knife or offset spatula around the edges of the pan to loosen the slab. Grasping the excess parchment, remove the slab from the pan and place on a cutting board. Cut into 9 or 12 pieces. Dollop the reserved whipped cream on top of each bar and dust with cocoa powder if desired.