In a (clean and grease-free) bowl of a stand mixer, on medium speed, beat the egg whites with the whip attachment until foamy. Add the cream of tartar and beat until the whites are at soft peak. Slowly add the sugar in a constant stream as the eggs are whipping. Add the vanilla, vinegar and cornstarch.
While the meringues are baking, make your lemon curd: In a large bowl, whisk together the yolks. Whisk in the sugar. Mix in the lemon juice and zest.
Over a double boiler, on medium heat, whisk the lemon mixture until it thickens about 172 degrees. Remove from heat, cool to 140 degrees (20 minutes). Whisk in butter.
In a bowl, beat the Mascarpone cheese, heavy cream, and vanilla. Gradually add sugar, and beat until it forms soft peaks.
Assembly: add a spoon of the chilled lemon curd into the well of the meringue bases, top with whipped mascarpone and fresh berries. Dust the pavlovas with powdered sugar and lemon zest. Be creative here--arrange and decorate so each of the elements are visible. Enjoy!