Heat oven to 250 degrees. Line a baking sheet with parchment paper. Draw a 7-inch circle on the parchment; flip paper. (For a crunchier Pavlova, opt for an 8-inch circle.)
In a small bowl, combine 3/4 cup of the sugar, the cornstarch and salt; set aside. Beat egg whites and cream of tartar on medium speed until soft peaks form, about 2 minutes. Increase speed to high; add sugar mixture 1 tablespoon at a time. Add vanilla and beat until mixture is glossy and sugar is incorporated, about 5 minutes.
Transfer egg-sugar mixture to parchment paper. Carefully spread within the circle, creating a slight dip in the middle (where raspberry sauce will rest). Bake at 250 degrees on middle rack for 1 hour. Turn off oven, crack door and cool inside for 1 hour.
Meanwhile, make raspberry sauce. In a small pot, combine 1 cup of the raspberries, remaining 1/4 cup sugar and 1 tablespoon water over medium heat. Bring to a simmer and cook 3 minutes, until sugar is dissolved. Stir in remaining 1 cup raspberries and pour into bowl to cool.
Transfer Pavlova to a plate; pour raspberry sauce on top and, if desired, garnish with mint. Whipped cream can also be spread on the Pavlova before sauce is poured on top.