Place the whipping cream for the ganache in a small pot and bring to a simmer. Mix in the chocolate and stir until the chocolate is completely melted. Mix in the 1 Tbsp. orange liqueur. Divide and pour the ganache into 4, 8 oz. flutes or wine glasses. Place the glasses in the fridge until the mousse is ready.
Place 1/2 cup orange juice in a medium-sized bowl. Sprinkle with the gelatine and let stand 5 minutes. Place another 1/2 cup orange juice in a small pot and bring to a boil. Add the boiling orange juice to the orange juice/gelatine mixture and mix until the gelatine is dissolved. Mix in the remaining 1 cup orange juice and 2 oz. orange liqueur.
Refrigerate the mixture for 45 minutes, or until it begins to thicken slightly, but is still fluid. Whip the 3/4 cup whipping cream and icing sugar until stiff peaks form. Whisk 1/2 of the whipped cream into the orange mixture to lighten. Fold in the rest of the whipped cream and the orange zest. Spoon the mixture into the glasses with the ganache. Cover and refrigerate for 2 hours, or until the mousse is set.
To serve, top each mousse with a little chopped chocolate; garnish with an orange slice and mint sprig.
Note: Knox Gelatine is available in the aisle where puddings are sold.