Melt the chocolate and put aside. Place the gelatin, orange juice and zest and cocoa powder in a small saucepan and heat gently until the gelatin dissolves. Next, whip the cream to stiff peaks. Mix 1/3 of the whipped cream into the melted chocolate quickly, then fold in the remaining 2/3 cream. Quickly fold in the melted gelatin mixture until you have a rich, light mousse. Crumble the muffins in a bowls and sprinkle a thin layer into the bottom of 8 small glasses. Spoon over some marmalade into each cup and top with half of the mousse mixture. Repeat the layering process again. Finally, decorate the top with some orange segment and chocolate shavings and allow to chill and set for 30 minutes before serving.