Preheat oven to 425°. In a large bowl, combine flour, sugar, baking cocoa, pie spice and salt. Stir in butter until crumbly. Press onto bottom and up sides of a 9-in. tart pan with removable bottom; place on a baking sheet. Bake 10 minutes; cool on a wire rack.
Meanwhile, in another large bowl, whisk pumpkin, eggs, brown sugar, orange zest, pie spice and salt. Slowly whisk in cream. Pour into crust. Bake 15 minutes. Reduce oven temperature to 350°. Bake until filling is set, 40-45 minutes. Cool on a wire rack.
For ganache, place chocolate chips in a small bowl. In a small saucepan, bring cream just to a boil. Pour over chocolate; let stand 10 minutes. Stir with a whisk until smooth. Spread over tart; chill until set. If desired, garnish with crystallized ginger.