In a medium saucepan, combine sweetened condensed milk and unsweetened chocolate; cook and stir over low heat until just boiling.
Reduce heat; cook and stir for 2-3 minutes more. Remove from heat.
Stir in vanilla and most of the graham cracker crumbs (reserve 2 Tbsp).
Spread evenly into foil-lined 8x8 inch pan.
Sprinkle with 1/3 c. mini chocolate chips, 1/3 c. Mallow bits, and remaining 2 Tbsp. graham cracker crumbs and press down gently into the bars.
Refrigerate until firm.
In a medium bowl, combine butter, cream cheese, vanilla, and powdered sugar and beat with electric mixer until smooth.
Spread over chilled bars; drizzle with melted chocolate and refrigerate 30 minutes or until firm.