Instructions

for the dough

Whisk egg yolk, cream, and vanilla together in a bowl. Process flour, cocoa powder, sugar, and salt together in food processor until combined, about 5 seconds. Scatter butter over top and pulse until mixture resembles coarse cornmeal, about 15 pulses. With processor running, add egg yolk mixture and continue to process until dough just comes together around processor blade, about 12 seconds.
Transfer dough to sheet of plastic wrap and form into 6-inch disk. Wrap tightly in plastic and refrigerate for at least 1 hour or up to 2 days. Let chilled dough sit on counter to soften slightly, about 10 minutes, before rolling. (Wrapped dough can be frozen for up to 1 month. If frozen, let dough thaw completely on counter before rolling.
Roll dough into 11-inch circle on floured counter, then transfer to parchment paper–lined rimmed baking sheet; cover loosely with plastic wrap and refrigerate until firm but pliable, about 10 minutes.
Loosely roll dough around rolling pin and gently unroll it onto 9-inch tart pan with removable bottom, letting excess dough hang over edge. Ease dough into pan by gently lifting edge of dough with your hand while pressing into corners and fluted sides of pan with your other hand. Run rolling pin over top of pan to remove any excess dough. Wrap loosely in plastic, place on large plate, and freeze until fully chilled and firm, about 30 minutes. (Dough-lined tart pan can be frozen for up to 1 month.) Adjust oven rack to middle position and heat oven to 375 degrees.
Line chilled tart shell with double layer of aluminum foil and fill with pie weights. Bake on foil-lined rimmed baking sheet until tart is set and fragrant, about 30 minutes, rotating sheet halfway through baking. Remove foil and weights and continue to bake 5 minutes longer. Transfer sheet to wire rack and let cool completely, about 30 minutes.

for the ganache

Bring cream, orange zest, and salt to simmer in small saucepan over medium heat. Off heat, add chocolate, cover, and let sit until chocolate is softened, about 5 minutes, then whisk to combine. Whisk in butter and vanilla until smooth. Pour filling into cooled tart shell, spreading into even layer with rubber spatula. Refrigerate, uncovered, until filling is chilled and set, at least 2 hours or up to 2 days.

for the jelly

Sprinkle gelatin over ¼ cup orange juice in bowl and let sit until gelatin softens, about 5 minutes. Cook orange zest and remaining ¾ cup juice, sugar, and salt in small saucepan over medium-low heat just until sugar dissolves, about 3 minutes, whisking occasionally. Off heat, add softened gelatin and whisk until dissolved. Strain mixture through fine-mesh strainer into bowl and let cool for 15 minutes; discard solids. Slowly pour orange mixture evenly over tart. Refrigerate until jelly is set, about 3 hours. (Tart can be refrigerated for up to 24 hours.)
Remove outer ring of tart pan, slide thin metal spatula between tart and tart pan bottom, and carefully slide tart onto serving platter or cutting board. Just before serving, cut away peel and pith from orange. Holding fruit over bowl, use paring knife to slice between membranes to release segments. Arrange orange segments attractively in pinwheel in center of tart. Serve.