In a mixing bowl, cream 1 1/2 cups peanut butter and powdered sugar until smooth. Roll into 30 3/4-inch balls. Cover and place in freezer on waxed paper. Only take out a few at a time. (The frozen peanut butter balls are much easier to work with.) Meanwhile, in a mixing bowl, cream butter, 1/2 cup peanut butter and sugars. Beat in egg whites and vanilla. Combine flour, cocoa and baking soda; gradually add to creamed mixture. Roll into approximately 125 1-inch balls. (You'll have plenty of extra dough for future cookies.)
Using floured hands, flatten chocolate balls and shape one around each peanut butter ball, sealing edges. Place 2 inches apart on greased baking sheets. Flatten with a glass dipped in sugar. Bake at 375° for 7 to 9 minutes or until cookies are set and tops are cracked. Cool for 1 minute before removing to wire racks.
For icing, in a small mixing bowl, cream shortening and powdered sugar. Beat in vanilla and enough milk to reach spreading consistency. Spoon into a resealable plastic bag or pastry bag; cut a small hole in corner of bag. Pipe icing over cookies in a zigzag pattern. Sprinkle with chocolate jimmies.