Mix the flour, cocoa powder, sugar, and salt in a bowl. Add the cold butter and work it in until the mixture looks like coarse crumbs. Stir in the egg yolk until a dough forms.

Press the dough evenly into a 9-inch tart pan. Chill it for 20 minutes, then bake at 350°F for 12 minutes. Let the crust cool completely.

Place the chopped dark chocolate in a heatproof bowl. Heat the heavy cream in a saucepan until it just starts to simmer, then pour it over the chocolate. Let it sit for 2 minutes.

Stir the chocolate and cream until smooth. Add the orange zest, orange juice, vanilla extract, and salt. Mix until fully combined.

Pour the filling into the cooled crust. Smooth the top and chill the tart for at least 2 hours until set.

Dust lightly with cocoa powder before slicing and serving.