Mix together the caster sugar and Truvia before gradually pouring into the whisked egg whites one spoon at a time. Mix until all of the sugar has been added and the meringue looks smooth, light and glossy. Fold in the vanilla extract and make into 12 small mounds on 2 baking sheets lined with baking parchment. Bake for 1 hour 30 mins before removing from the oven and leaving to cool completely on a wire rack.