1.Preheat the oven to 200° and have ready a parchment-lined baking sheet. In a bowl of a stand mixer with whisk attachment add the cold egg white and cream of tartar. Beat with mixer until foamy and soft peaks start to form.
2.Then, slowly stream in the granulated sugar while continuing to beat on medium speed. When it has stiff peaks, stop mixing. Use two spoons to make four circles of meringue on the baking sheet. Use a spoon to make a slight divot in each circle to hold the fillings later.
3.Bake for 45 minutes, and then turn off oven and let cool for 1 hour. Do not open the oven during the cooling process. The meringues should not appear sticky to the touch, though the insides may be.
4.After cooling the meringues, move them to serving plates. Pour the whipping cream, powdered sugar and vanilla into a bowl of a stand mixer with whisk attachment. Beat together at medium speed then add the vanilla when the cream has soft peaks. Whip until whipped cream has formed.
5.Add a dollop of the whipped cream on top of each meringue shell. Top with the mixed berries and serve.