Preheat the oven to 250 degrees and line a baking sheet with parchment paper.
In a clean bowl, beat the egg whites until soft peaks form. Gradually add the granulated sugar and continue beating until stiff, glossy peaks form.
Gently fold in the cornstarch, lemon juice, and half the lemon zest.
Spoon the meringue into four small nests on the prepared sheet. Bake for 60 minutes, then turn off the oven and let the pavlovas cool inside with the door closed.
In a separate bowl, stir the mascarpone with the powdered sugar, vanilla, and remaining lemon zest until smooth.
Once the pavlovas are cool, spoon the mascarpone mixture into the center of each one and top with blueberries.