1. In food processor, pulse together flour, cocoa powder, sugar and salt to blend. Add cold butter; pulse until coarse crumbs form and no large pieces of butter remain. Add egg yolk and the cold water; pulse until dough begins to come together. Transfer dough to sheet of plastic wrap. Shape into disk; wrap disk with plastic. Refrigerate until firm, 30 minutes.
2. Once dough has chilled, heat oven to 425 degrees F. Using a paper towel lightly moistened with vegetable oil, lightly grease edge of 10-inch pie plate.
3. Roll out dough between 2 sheets of plastic wrap into 14-inch circle. Remove top sheet of paper; invert crust into pie plate and remove paper. Gently press crust over bottom and up sides of pie plate. Crimp edge of crust as desired. Prick crust all over with fork. Place in freezer for 10 minutes to firm.
4. Line crust with foil; fill with dried beans or pie weights.
5. Bake in 425 degrees F oven for 10 minutes. Carefully remove from oven. Remove foil and pie weights. Return piecrust to oven; bake until set and dry to touch, another 8 to 10 minutes. Let crust cool in plate on wire rack.Filling:
6. In small saucepan, stir together gelatin, 1/3 cup sugar, the salt, water and yolks. Let stand for gelatin to soften, about 3 minutes. Heat over medium-low heat, whisking gently, until mixture is slightly thickened and registers 160 degrees F on instant-read thermometer, about 8 minutes. Strain mixture through fine-mesh sieve into large bowl. Stir in vanilla. Refrigerate 5 minutes.
7. In large bowl, beat cream until soft peaks form. Refrigerate. Remove gelatin mixture from refrigerator; whisk slightly. Let stand at room temperature.
8. In clean medium-size bowl, beat together powdered egg whites and warm water with clean beaters just until foamy. Gradually add remaining 1/3 cup sugar and beat until shiny, stiff peaks form.
9. Fold beaten whites into gelatin mixture. Fold in chilled whipped cream. Remove 1-1/2 cups of mixture to small bowl and fold in dissolved coffee. Dollop 1 cup coffee-flavored chiffon into cooled piecrust. Top with vanilla chiffon. Place small dollops of the remaining coffee chiffon on top. Swirl with thin knife or spatula to marbleize chiffon. Refrigerate, covered, until filling is set, about 2 hours (cover by inverting large bowl over top of pie).
10. Before serving, garnish with chocolate curls, if desired.