To Make the Lemon Custard: In medium pot, bring milk to boil, then reduce to simmer. Add cornstarch (dissolved in water), and sweetener, stir for one minute to thicken. Meanwhile, temper egg yolks, then slowly add to the pot, and whisk for about 2 minutes over medium high heat. Turn off heat and whisk in the lemon zest, lemon juice, and vanilla. Put in a bowl in fridge to chill completely, or at least one hour.
To Make the Trifles: Remove from fridge and fold in whipped topping. Layer custard between layers of berries in small jars or clear glasses. Refrigerate until ready to serve!
Note: You may also make this in a medium trifle dish, or a medium clear bowl, but if you are making this for a large crowd, I recommend doubling the recipe, or adding layers of cake or cookies to expand the servings.