Preheat the oven to 250 degrees F (120 degrees C). Line a baking sheet with parchment paper or aluminum foil.

Beat egg whites, cream of tartar, and salt in a large bowl with an electric mixer until soft peaks form. Beat in sugar, 1 tablespoon at a time, until stiff, glossy peaks form. Mix in cornstarch, vinegar, and vanilla until blended.

Spread egg white mixture on the prepared baking sheet into a 10-inch (25cm) circle with a raised edge and a slight indentation in the center.

Bake in the preheated oven until meringue is cream-colored and firm to the touch, about 1 hour 15 minutes. If meringue appears to be browning, reduce the oven temperature to 225 degrees F (105 degrees C). Remove meringue from the oven and let cool.

When ready to serve, peel parchment paper off the bottom of meringue. Place meringue onto a serving plate.

Whip cream in a medium bowl with an electric mixer until stiff peaks form. Gently stir in liqueur until incorporated.