In a small saucepan, whisk sugar, cornstarch, cocoa powder and salt together until combined. Slowly stream in half & half and milk, whisking to prevent lumps. Place over medium high heat and whisk until mixture starts to steam. Sprinkle in chopped dark chocolate and continue stirring until it has melted and the pudding has bubbled and thickened. Remove from heat and stir in vanilla bean paste. Pour into 4 servings dishes. Place plastic wrap directly onto pudding and refrigerate until completely chilled, approximately 2 hours. Remove plastic wrap and top with sweetened whipped cream and dark chocolate.