Whisk flour and salt together. Cut in cold butter until crumbly. Add ice water gradually until dough forms. Shape into disc, wrap, and chill 30 minutes.
Roll dough to 12-inch circle and fit into 9-inch pie dish. Crimp edges, prick bottom with fork, line with parchment and pie weights. Bake at 400°F for 15 minutes, remove weights, bake 8-10 minutes more until golden. Cool completely.
Sprinkle gelatin over ¼ cup cold water and let stand 5-10 minutes until bloomed (wrinkled and firm).
Whisk milk, cocoa powder, ⅓ cup sugar, and salt in saucepan over medium heat until smooth and steaming. Temper egg yolks by slowly whisking in ½ cup warm milk mixture, then return to saucepan.
Cook custard over medium-low heat, stirring constantly, 3-4 minutes until thickened to coat spoon. Remove from heat, add bloomed gelatin and melted chocolate, whisk until smooth. Stir in vanilla.
Strain custard through fine mesh sieve into bowl. Cover surface with plastic wrap. Cool at room temperature 20 minutes, then refrigerate 20-30 minutes until thick but not set.
Combine egg whites, remaining ⅓ cup sugar, and cream of tartar in heatproof bowl over simmering water. Whisk constantly until reaches 160°F and sugar dissolves, 3-5 minutes.
Remove from heat and beat with electric mixer on high until glossy stiff peaks form, about 4-5 minutes.
Stir one-third of meringue into cooled chocolate custard. Gently fold in remaining meringue in two batches until no white streaks remain, being careful not to deflate.
Pour filling into cooled crust, smooth top. Refrigerate uncovered at least 4 hours until fully set, preferably overnight.
Beat cold heavy cream and powdered sugar in chilled bowl with mixer on medium-high until soft to medium peaks form, 2-3 minutes. Spread or pipe over chilled pie.
Garnish with chocolate shavings. Slice with knife dipped in hot water and wiped clean between cuts. Serve chilled.