Cut off the top third of each orange and discard.

Scoop out the pulp from the oranges, reserving 1 tablespoon of juice and discarding the remaining pulp.

Set the hollowed orange shells aside.

Combine chocolate, Grand Marnier, reserved orange juice, and grated orange peel in a heatproof bowl.

Place the bowl over a pot of gently simmering water and melt the chocolate mixture, stirring until smooth.

Remove the bowl from the heat and stir in the egg yolks one at a time.

In a separate bowl, use an electric mixer to beat the egg whites with cream of tartar until soft peaks form.

Gradually add sugar to the egg whites, continuing to beat until stiff but not dry peaks form.

Gently fold 1/4 of the beaten egg whites into the chocolate mixture to lighten it.

Fold in the remaining egg whites, being careful not to deflate the mixture.

In another bowl, beat the whipping cream until stiff peaks form.

Fold the whipped cream into the chocolate mixture until well combined.

Spoon the mousse into the prepared orange shells.

Refrigerate the mousse-filled orange shells until firm, approximately 2 hours.

Garnish with candied orange peel before serving.