1. Preheat the oven to 300˚F (150˚C). Prepare a cookie sheet by lining it with parchment paper. On the paper, using a small bowl or coffee mug (3.5” (9 cm) in diameter) trace 6 evenly spaced circles to use as a guide for shaping meringue. Place parchment paper pencil-mark side down on the cookie sheet.
2. In a large, clean and grease-free bowl, using an electric mixer and the whisk attachment, beat egg whites on medium speed until foamy. While the mixer continues to run, add sugar gradually, 1 tbsp (15 mL), at a time. Once all of the sugar is added, increase the mixer speed to high. Continue beating until mixture becomes white and glossy. When soft peaks form, reduce speed to low and add cream of tartar and vanilla. Once incorporated, increase speed to high and continue beating until stiff peaks form. Spoon some of the mixture onto each circle on the parchment paper, building up the sides to form shells.
3. Bake for 30 minutes or until the meringue exteriors appear dry and is crisp throughout. Turn off oven and let shells cool in the oven with the door closed about 1 hour. If making ahead, store meringues in an airtight container on the counter or in a cool place (do not store in the fridge or freezer).
4. Before serving, whip cream until soft peaks form. Add icing sugar and continue beating until stiff peaks form. If using, fold in Irish cream liquor. Spoon whipped topping into meringue shells and top with berries.