Combine cracker crumbs, sugar, cocoa powder and melted butter in a medium-size bowl. Stir until evenly combined. Press into bottom and up sides of a 9-inch removable-bottom tart pan. Refrigerate until ready to fill.
In small saucepan, combine chocolate and 1 cup of the heavy cream. Heat over medium heat, stirring until smooth. Remove from heat; stir in mint extract. Set aside 1/2 cup. Pour remainder into cold crust. Freeze 15 minutes.
In glass bowl, sprinkle gelatin over 3 tablespoons cold water. Let sit 5 minutes. Microwave 20 seconds, until melted. Let cool. Meanwhile, combine cream cheese, milk and 4 tablespoons of the sugar in a large bowl. Beat until smooth. Slowly add reserved 1/2 cup chocolate mixture. Beat until smooth. Beat in 2 tablespoons of the gelatin mixture. Fold in mint pieces. Spoon into crust over chocolate layer, spreading level. Freeze 5 minutes.
Beat remaining 1 cup cream and 3 tablespoons sugar in bowl until soft peaks form. With mixer running, blend in remaining gelatin mixture and beat to firm peaks. Spread on top of tart. Refrigerate for at least 2 hours. Garnish, remove side of pan and serve chilled.