Preheat the oven to 350°F.

To make the filling, melt the chocolate and butter together in a bowl set over simmering water. When melted, remove from the heat and stir to combine. Set aside to cool.

Combine the sugar, flour, and eggs in a mixing bowl and whisk until well blended. Stir in the chocolate-butter mixture. Pour the filling into the tart shell, and bake for approximately 1-1/4 hours, until the filling is set. Remove to a rack and let cool completely

TO MAKE THE ORANGE SAUCE, combine the orange juice, Grand Marnier, and sugar in a heavy, nonreactive saucepan and cook over low heat, stirring constantly, until thick and syrupy and reduced by half Remove from the heat, stir in the grated orange zest, and let cool.

To serve, place a piece of the tart on a plate and spoon prepared Crème Anglaise around the bottom. Drizzle a small amount of orange sauce into the crème anglaise in swirls.