Preheat the oven to 350 degrees F. Place the tart shells on a parchment-lined baking tray; divide the chocolate chips and walnut pieces among them. Beat the eggs in a bowl; mix in the corn syrup, brown sugar, butter and vanilla. Pour the mixture into the tart shells; filling them right to the brim. Bake 25 minutes. Cool on a baking rack to room temperature. Chill in the fridge; remove the foil liners, set on serving tray and dust with icing sugar, if desired, and serve.