Preheat oven to 350°.
To prepare crust, place first 4 ingredients in a large bowl; beat with a mixer at medium speed until smooth. Lightly spoon 1 cup flour into a dry measuring cup; level with a knife. Add flour and salt to peanut butter mixture; cut in flour with a pastry blender or 2 knives until mixture resembles coarse meal. Sprinkle surface with cold water, 1 tablespoon at a time; toss with a fork until combined.
Shape mixture into 24 balls. Place 1 ball in each of 24 miniature muffin cups coated with cooking spray. Press dough into bottoms and up sides of muffin cups.
To prepare filling, combine 2/3 cup brown sugar and the next 4 ingredients (2/3 cup brown sugar through milk) in a small saucepan over medium-low heat. Cook 3 to 4 minutes or until smooth, stirring frequently. Remove from heat; stir in 2 tablespoons flour and egg until well blended. Divide chocolate mixture evenly among muffin cups. Bake at 350° for 10 minutes or until the pastry is lightly browned; cool in pan on a wire rack 5 minutes. Run a knife around outside edges of cups. Remove cups from pan; cool completely on wire rack. Sprinkle with powdered sugar.