Prepare the oven and pan. Preheat the oven to 300°F. Line a baking sheet with parchment paper. Trace an 8-inch circle onto the parchment using a cake pan or plate as your guide, then flip the parchment over so the marking is underneath. You'll still be able to see the circle through the paper without getting pencil or ink on the meringue.
Make sure your equipment is completely clean. Before whipping the egg whites, check that your bowl and whisk are completely clean, dry, and free of any grease. Even a small amount of fat or egg yolk can interfere with the egg whites whipping properly.
Begin whipping the egg whites. Add the room-temperature egg whites and a pinch of kosher salt to the bowl of a stand mixer fitted with the whisk attachment. Beat on medium speed until the whites become foamy and begin to hold very soft peaks. Add the cream of tartar and continue beating until soft peaks form. When you lift the whisk, a peak should form but flop back over rather than standing upright.
Gradually add the sugar. Increase the mixer to medium-high. Add the sugar approximately 1 tablespoon at a time, waiting about 15–20 seconds between additions. Don't rush this step. Gradual incorporation helps the sugar dissolve properly and creates a more stable meringue.
Whip to stiff, glossy peaks. After all of the sugar has been added, continue whipping until the meringue becomes thick and glossy and holds stiff peaks. When you lift the whisk, the peak should stand upright rather than collapsing.
Rub a small amount of meringue between your thumb and finger. It should feel mostly smooth with little to no noticeable sugar grit. If it still feels distinctly grainy, continue whipping briefly and test again.
Add the remaining ingredients. Sprinkle the cornstarch over the meringue and add the vanilla. Mix on low for only a few seconds, then use a rubber spatula to gently finish incorporating everything by hand. Avoid aggressively mixing at this stage because you don't want to knock excess air out of the meringue.
Shape the pavlova. Spoon the meringue onto the center of your traced circle. Use an offset spatula or the back of a large spoon to shape it within the 8-inch guide, building upward rather than spreading outward. The finished pavlova should be approximately 2½–3 inches tall.
Create upward swoops around the sides, then make a shallow well in the center, leaving a substantial raised edge. The well will eventually hold the mascarpone cream and pears, but don't hollow the center all the way down.
Bake. Place the pavlova in the preheated oven and immediately reduce the oven temperature to 250°F. Bake for 1 hour 20 minutes.
Cool slowly. When the baking time is complete, turn the oven off but do not open the door. Leave the pavlova inside the closed oven for at least 2 hours, or until completely cool.
Once cooled, the exterior should feel dry and crisp. The center will remain soft and marshmallow-like. Some cracking around the exterior is completely normal.