Lightly coat an 8x8x2-inch baking pan with cooking spray.* Set aside.
For crust, place oats in a food processor; cover and pulse with several on/off turns until finely ground. In a medium saucepan combine 1/4 cup of the chocolate pieces and the butter; heat and stir over low heat until melted. Stir in oats, crushed wafers, cocoa powder, milk, and salt. Press crust mixture into prepared pan. Chill for 15 minutes.
In the same saucepan combine the remaining 1/4 cup chocolate pieces and the shortening. Heat and stir over low heat until melted. Set aside.
In a medium bowl combine powdered sugar, cream cheese, and peppermint extract; stir until smooth. Spread evenly over crust. Drizzle with the melted chocolate mixture.
Chill about 1 hour or until chocolate drizzle is set (cream cheese layer will remain soft). If desired, sprinkle with fresh mint. Cut into 24 bars.
Tip
*Tip: To make removing the bars easier, line the baking pan with foil in Step 1, extending foil over the edges of the pan. Using the edges of the foil, lift the uncut bars out of the pan in Step 5. Cut into bars.