Preheat oven to 350 degrees.  Lightly grease ramekins (see note about sizes) and set on a baking sheet.

Melt chocolate and butter using a double boiler, stirring until melted and smooth.  Remove from heat.  In a separate bowl, whisk together eggs and sugar until thick and frothy, about 1 minute.  Slowly drizzle chocolate mixture into egg mixture while whisking constantly.  Stir in orange zest.

Transfer batter to ramekins, filling 2/3 full.  Bake for 25-35 minutes, until tops are set and a skewer poked in cakes comes out moist and slightly fudgy but not wet. 

While tarts are cooling a bit, prepare ganache.  Place cream in heat proof container and heat in microwave until hot and starting to bubble.  Remove from microwave and stir in chocolate until melted and smooth.  

place about 2 teaspoons ganache in center of each tart, set the rest aside to drizzle on top.  Tarts can sit like this at room temperature all day if needed.  Prepare whipped cream before serving.  Whip cream, powdered sugar and extract until soft peaks form.  Add zest and continue whipping until desired consistency is reached.  Spoon whipped cream on top of tarts and drizzle with extra ganache (you can add a little more warm cream to it if you'd like to thin for drizzling.)