In large bowl, whisk together flour, cocoa powder, 2 tbsp (30 mL) sugar and salt. Cut in cold butter with pastry-blender or rub with fingertips until mixture resembles coarse meal. In cup, use fork to beat egg yolk with 3 tbsp (45 mL) cold water. Stir into flour mixture until dough comes together. Stir in 1 tbsp (15 mL) additional cold water, if needed. Form dough into disc. Wrap in plastic; chill at least 1 hr.
Preheat oven to 400°F (200°C). Roll dough to fit into 9-in. (23-cm) tart pan with removable bottom. Drape dough over pan; gently pressing into bottom and up sides. Use rolling pin to roll over rim of pan to trim dough flush with rim. Chill 15 min. Using fork, prick bottom of tart shell all over. Line shell with foil; fill with pie weights or dried beans. Place on parchment paper-lined baking sheet. Bake 15 min. to "blind bake". Carefully remove foil and weights. Reduce heat to 350°F (180°C). Bake shell additional 5 to 10 min. until pastry is firm and crisp. Cool completely.
Place chocolate in heat-proof bowl. In saucepan, bring cream and orange liqueur just to a boil over medium-high heat. Pour onto chocolate; let stand 1 min. Whisk in room temperature butter until chocolate is completely melted. Pour into shell. Chill 3 hr., or until filling is set.
Meanwhile, set clementine rounds on paper towel-lined baking sheet to absorb excess juices. Place clementines in single layer on rack set on foil-lined baking sheet. Sprinkle rounds evenly with remaining 2 tbsp (30 mL) sugar. Place under hot broiler about 2 min. until sugar begins to turn golden brown. Watch carefully to avoid burning. Let sugar cool and harden; arrange clementines in ring of overlapping rounds on tart.